The interpretation timeline
Isaiah 19:20 Douay-Rheims

It shall be for a sign, and for a testimony to the Lord of hosts in the land of Egypt. For they shall cry to the Lord because of the oppressor, and he shall send them a Saviour and a defender to deliver them.

A.D. 317 – 1871 8 witnesses 9 excerpts

Patristic

before A.D. 750
317
A.D.
c. A.D. 240–317
“He was called "Christ" from the anointing. Then, that the same one was a human being Jeremiah shows, saying, "And he is man, and who has known him?" And Isaiah, "And the Lord shall send them a man, who shall save them, and judging them he will heal them."”
“That he was both God and man was declared before by the prophets. That he was God, Isaiah thus declares, "They shall fall down before you, they shall make supplication to you, since God is in you, and we knew it not, even the God of Israel. They shall be ashamed and confounded, all of them who oppose themselves to you, and shall go unto confusion." … Likewise that he was man … Isaiah also thus speaks, "and the Lord shall send them a man who shall save them, and with judgment shall he heal them."”
Source
397
A.D.
Ambrose of Milan Patristic
A.D. 339–397
“He said, I do not have a man; that is, because through a man is death, and through a man is the resurrection of the dead. He could not descend, he could not be saved, who did not believe that our Lord Jesus had taken flesh from the Virgin. But this one who was waiting for the mediator between God and men, the man Jesus Christ, expecting him of whom it was said: And the Lord will send a man to save them; he said: I do not have a man; and therefore he deserved to come to health, because he believed in the one who was coming.”
Source

Scholastic

c. 1100 – 1500
1105
A.D.
Rashi Jewish
1040–1105
“And it shall be The altar shall be for a sign and for a witness between them and the Omnipresent. a savior and a prince Heb. מוֹשִׁיעַ וְרָב, a savior and a prince.”
1167
A.D.
Ibn Ezra Jewish
1089–1167
“They shall cry unto the Lord because of the oppressors, etc. Whosoever will be in trouble will come thither to bring his offering. ורב And a great one.: a deliverer who is also a great man; a great deliverer. It would be a more natural explanation to take רב as participle of ריב to plead; a helper and a defender. Comp. רב chief (Est. 1:8)”
Source
1256
A.D.
Thomas Aquinas Catholic
1225–1274
“The burden of Egypt. In this part he threatens the destruction of the Egyptians, following out its manner and order; and it is divided into three parts: first, the preparation of the punishment is described; in the second, the destruction itself, at "and I will deliver Egypt"; in the third, the resulting benefit, at "in that day there shall be five cities." Now the preparation of the punishment is described as to four things. First, as to the disposition of the enemies: "he shall ascend" — by inspiring — "upon a cloud," from which comes the storm; "light," on account of agility and readiness: Ps. 96: "cloud and darkness are round about him." Second, as to the stupefaction of the ruling gods. "And they shall be moved": because they were broken by the Assyrians; or because they were not able to give an answer; or because they did not even appear to man on that same day. Ez. 6: "they shall be broken," etc. "The heart of Egypt," that is, the king and the princes, from whom all the vigor of Egypt comes. Above, 1: "every head is languishing, and every heart sad." Third, as to the dissension of those resisting: as to outward engagement, "to clash"; as to strangers, "the Egyptians"; as to kinsmen, "man shall fight"; as to acquaintances and friends, "and man against his friend"; as to different peoples, "city." He sets down the diversity of wills: "it shall be broken up," that is, divided, "the spirit," that is, the will, "in its bowels," that is, in the inhabitants of the land. Matt. 12: "every kingdom divided against itself shall be made desolate." Fourth, as to the error of men taking counsel: "and the counsel." Job 5: "he catches the wise in their own craftiness, and scatters the counsel of the wicked" — and of the demons: "and they shall inquire" — in vain. Above, 8: "and when they shall say to you: seek unto pythons, and unto wizards who mutter in their enchantments," etc. Note, concerning that word "upon a cloud," that there is a manifold cloud. The first is of the flesh assumed. Job 38: "Where were you when I made a cloud its garment, and wrapped it in darkness as in swaddling clothes of infancy?" The second is of the virginal womb. 3 Kings 18: "Behold a little cloud arose out of the sea." The third is of the devout mind. Ps. 67: "His magnificence and his power are in the clouds." The fourth is of the seat of judgment. Ps. 96: "Cloud and darkness are round about him; justice and judgment are the preparation of his seat." Also note that from the first cloud proceeds the sacramental or sanctifying rain. John 19: "from the side (of the Lord) immediately came out blood and water." From the second proceeds the food of consolation. Wis. 19: "the cloud overshadowed the camp, and out of the water that was before, dry land appeared." And below: "seeing your wonders and marvels: for like horses they fed on the food." Above, 18: "like a cloud of dew." From the third proceeds the flashing of preaching. Job 37: "the clouds long for the grain, the clouds scatter their light." From the fourth proceeds the storm of damnation. (Jer. 4): "behold as a cloud"; Ps. 67: "his power is in the clouds." Also note, concerning that word "a light cloud," that the first is called light on account of the dignity of its eminence. Daniel 7: "behold, one like the son of man came in the clouds." The second is called light on account of the purity of virginity. Apoc. 14: "and behold a white cloud, and upon the cloud one sitting like to the son of man." The third is called light on account of the swiftness of obedience. Below, 60: "who are these that fly as clouds?" The fourth is called light on account of the ease of its execution. Ecclus. 43: "in his greatness he set the clouds, and the stones of hail were broken." "And I will deliver Egypt." Here he pursues the destruction itself: first, as to the servitude of the people; second, as to the desolation of the land, "and it shall be dried up"; third, as to the stupefaction of the wise men, "the princes are fools." "Of masters" — of the Assyrians, the Chaldeans, the Romans. Deut. 28: "you shall serve your enemy." Now he designates the destruction of the land through the dryness of the waters, in which Egypt is adorned and rich: either because the wrath of God brought it about beforehand that the waters should dry up, and the animals die; or he speaks hyperbolically, as if to say: the tribulation will be so great that it will seem even to dry up the waters. He sets down, therefore, three things. First, the manner of the drought: "it shall be dried up," that is, diminished, "the river" — the Nile — "the streams," the other rivers. Ezechiel 30: "I will make the channels of the rivers dry." Second, he sets down the sign of the drought: "the reed and the bulrush," which are nourished in the waters. Below, 35: "the greenness of the reed and the bulrush shall spring up." Third, he sets down the harm of the drought: "it shall be made bare" — first, of agriculture, "from the fountain," that is, from the beginning; or this refers to the end. Ps. 106: "he turned rivers into a desert, and the outgoings of waters into thirst." Second, of the fishermen: "the fishermen shall mourn." Ezech. 29: "I will cast you into the desert, and all the fish of your river." Third, of the various craftsmen: "they shall be confounded who work in flax," which grows in moist places. "Withering," that is, drying up; "ponds," fish-ponds. "The princes are fools." Here he sets down the stupefaction of men: first, as to error in counsels; second, as to fear in wars, at "in that day." Concerning the first, three things. First he mocks the folly of the wise as to deliberation: "the princes are fools" — that is, the philosophers — "of Tanis," where study flourished: Jer. 51: "every man has become foolish"; as to the promulgation of counsel: "the wise"; as to their own self-commendation: "how will you say to Pharaoh?" That is, how will you be able to say, when he will think that you have counseled badly, this thing which each one of you was accustomed to say for his own commendation, so that his counsel might be believed: "I am the son of the wise" — as if to say, I possess wisdom by inheritance; or "a disciple of the kings" — who of old were philosophers. Job 15: "both the aged and ancient men are among us, much elder than your fathers." Second, he mocks the confidence of the king: "Where now?" And first, the question: below, 47: "let them stand and save you"; second, the answer: "the fools have withered away," that is, have failed, as above; third, he mocks the cowardice of the deceived people: "they have deceived." Here he does three things. First he sets down the deception of the wise: "they have deceived the corner," that is, the kingdom, in which many are joined together as walls at a corner. Below, 47: "your wisdom and your knowledge have deceived you." Second, the permission of God: "the Lord has mingled" — this is often said metaphorically. "A spirit of dizziness," that is, of error, after the manner of one who suffers vertigo. 3 Kings, last chapter: "the Lord has put a lying spirit in the mouth of all your prophets." Third, the error of the people itself: "and to err." "Head" — namely, neither as to the beginning; "tail" — neither as to the end; "bowing down" — as to the old man; "holding back" — as to the young man, who needs restraint, or one given to wantonness. 2 Tim. 3: "but evil men and seducers shall grow worse." "In that day." Here he threatens the fear of the strong. And first he sets down the fear: "as women," weak and feeble. Jer. 51: "their strength is devoured, and they have become as women." Second, the reason: "the land of Judah shall be" — namely, the memory of the evils which they did to them, turning them away from the worship of God, on account of which they shall know that they have incurred the wrath of God. 1 Mach. 6: "now I remember the evils that I did in Jerusalem." "In that day." Here the resulting benefit, or consolation, is set down — namely, their conversion. And first, a threefold sign of the conversion: of which the first is the oath, "swearing." Below, 65: "he who swears," etc., "shall swear by the God, Amen: because the former distresses are delivered to oblivion." "The city of the sun" — Heliopolis. "The language of Canaan" — which borders on the language of the Jews, so that they might also agree in language with the people of God. For among these certain Syrians were transplanted by Nebuchadnezzar, who kept their own language. The second sign is a sacred building: "an altar of the Lord, and a monument" — because they worshiped many gods, as is said in Acts 17. This (some say) was fulfilled in the time of the Maccabees, when, many Jews fleeing into Egypt, Onias, wishing to fulfill this prophecy, built an altar against the precept of the Law, which is against that word of Exod. 20: "you shall not build me an altar," etc.; concerning which it is argued in Dan. 11: "the children also of the transgressors of your people shall lift themselves up to fulfill the vision, and they shall fall." Whence it is clear that this is not the meaning, but rather it concerns an altar to be built for Christian worship. The third sign is that they call upon the help of God: "they shall cry out"; "a savior" — Jesus. Ps. 90: "he cried to me." Second, he sets down the manner of the conversion: first, as to the reception of the faith, "and he shall be known": Jer. 31: "all shall know me, from the least even to the greatest, says the Lord"; second, as to the offering of sacrifices and vows, "and they shall worship": Malachi 1: "in every place there is offered to my name a clean oblation"; third, as to the remission of sins: "and he shall strike" — first, through many punishments; "and he shall heal" — from sin. Job 5: "he wounds and cures; he strikes, and his hands shall heal." Third, he sets down the threefold effect of the conversion: namely, peace: "in that day there shall be a way," which before could not be, on account of the different kingdoms, which at the birth of the Lord were joined together under the Romans. "They shall serve" — in trading with one another; or because Syrian soldiers were in the legions of the Romans for the guarding of Egypt. Above, 2: "nation shall not lift up sword against nation." Second, he promises the second effect, the truth of the faith: "in that day the third" — together, and serving the Romans equally, and in unity of faith. John 10: "there shall be one fold and one shepherd." Rom. 10: "there is no distinction of Jew and Greek." Third, he sets down the third effect, namely the divine blessing: "it shall be" — namely, Israel — "a blessing," because through the apostles the whole world received the blessing of God. "An inheritance," because he was born there, and fulfilled the mysteries of our salvation there. Ps. 113: "he blessed the house of Israel."”

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“And it shall be for a sign and for a witness unto the Lord of hosts in the land of Egypt,.... This refers either to what goes before, that the altar and pillar were signs and witnesses that the Lord was believed in, professed, and worshipped there; or to what follows after, that the Lord's hearing the cries of men, and answering them, by sending a great Saviour to them, is a token and testimony for him of his great love unto them: for they shall cry unto the Lord because of the oppressors; as men awakened and convinced do, feeling the oppressions of a guilty conscience, and a tempting devil, and an ensnaring wicked world: and he shall send them a Saviour, and a great one, and he shall deliver them; this is Christ, whom God sent in the fulness of time to be the Saviour of lost sinners; and he is a "great" one indeed, the great God, and our Saviour, Tit 2:13 who is the Son of God, the true God, and eternal life, who has all the perfections of deity in him; the Creator and Upholder of all things; and must have therefore great and sufficient abilities to save sinners to the uttermost; and those that come to God by him he does save and deliver from all their sins, and out of the hands of all their enemies, and from wrath, ruin, and destruction. Abarbinel (e) owns that the Messiah is here meant, as undoubtedly he is; and not the angel that destroyed Sennacherib's army, as Kimchi; for the text speaks not of the Jews, but of the Egyptians. Vitringa thinks that either Alexander, called the Great, or else Ptolemy the son of Lagus, who had the same epithet, and who was also called "Soter", the saviour, is here meant. (e) Mashmiah Jeshua, fol. 13. 1.”
Source
1849
A.D.
1774–1849
“Them. The Jews were miraculously rescued from the hands of Philopater, (Josephus, contra Apion ii.) or rather Christians are delivered from sin and Satan.”
1871
A.D.
1871
“it--the altar and pillar. a sign--(of the fulfilment of prophecy) to their contemporaries. a witness--to their descendants. unto the Lord--no longer, to their idols, but to Jehovah. for they shall cry--or, "a sign . . . that they cried, . . . and He sent to them a saviour"; probably, Alexander the Great (so "a great one"), whom the Egyptians welcomed as a deliverer (Greek, Soter, a title of the Ptolemies) out of the hands of the Persians, who under Cambyses had been their "oppressors." At Alexandria, called from him, the Old Testament was translated into Greek for the Greek-speaking Jews, who in large numbers dwelt in Egypt under the Ptolemies, his successors. Messiah is the antitype ultimately intended (compare Act 2:10, "Egypt").”
Source
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.