Rev 3:7

“These things, moreover, as I judge, he gives forth only and truly by participation in the divine Spirit. There is need of the same power for those who prophesy and for those who hear the prophets. No one can rightly hear a prophet unless the same Spirit who prophesies bestows on him the capacity of grasping his words. And this principle is expressed indeed in the holy Scriptures themselves when it is said that only he who shuts opens, and no other one whatever; and what is shut is opened when that word of inspiration explains mysteries.”

“"Who has the key of David," that is, royal authority. He says "the key of David" either because Christ was born from his family or because David himself foretold many things concerning Christ. Through the dispensation of Christ, as though by a key, the secrets of the law and of the prophets and of the psalms were made manifest. Christ himself attests to this when he says, "Everything written about me in the law of Moses and the prophets and the psalms must be fulfilled."”

“"Who opens and no one shall shut, who shuts and no one opens." It is evident that Christ opens [the door] to those who knock, but to hypocrites who knock and say, "Lord, Lord open to us," he closes the door of life and says to them, "I do not know from where you come; I never knew you, you evildoers."”

“The secrets of the divine law are disclosed to the faithful by the power of Christ alone, and they are closed to the unbelievers. However, when he loosens, no one shuts, and when he binds, no one loosens.”

“His kingdom is called "the key of David," for this is a symbol of authority. And, moreover, the Holy Spirit is the key of the book of the Psalms and of every prophecy, for through him the treasures of knowledge are opened. He received the first key according to his humanity, but the second key he possesses according to his deity, which has no beginning. Since in some of the copies there is written "hades" instead of "David," through the image of the "key of hades" the authority of life and death is ascribed to Christ.”

“And to the angel of the church in Philadelphia write. Philadelphia is interpreted as brotherly love, which is promised the open door of the kingdom and to be loved by the Lord.”

“These things says the holy one, the true one, who has the key of David. That is, the royal authority, whether born from the lineage of David or because the prophecy of David was revealed through the dispensation of Christ.”

“Who opens and no one shuts, etc. The secrets of the divine law are revealed to the faithful by the power of Christ alone and are closed to the unfaithful.”

“And to the angel of the church of Philadelphia, write: These things saith the Holy One and the true one, he that hath the key of David; he that openeth, and no man shutteth; shutteth, and no man openeth: Behold, I have given before thee a door opened, which no man can shut. The Lord is called the Holy One and the True One as if he were the only one, because compared to him no one is holy or truthful. It is indeed written, No one is clean from filth, [Variant of Job 14:4, very close to the LXX version.] as well as, Every man is a liar. [Ps. 115:11] We take the key of David to mean our Redeemer's incarnation, which he got from the seed of David. The door, which the same key opens and shuts, is Christ himself, according to this: I am the door, etc. [John 10:9] For he is called the key and the door in the same way as in the Gospel he is called the shepherd as well as both the porter and the door. [Ibid.1-5] So the open door is Christ born according to the flesh, having suffered, and resurrected on the third day; the door however is not open to the incredulous, but to believers. The Jews were trying to shut this door when they scourged the apostles and charged them that they should not speak in the name of Jesus; [Acts 5:40] but since when he opens, no man shuts, they replied immediately, We cannot but speak the things which we have heard. [Acts 4:17-20] As this door was shut to the Asians, the apostles tried to open it; but since when he shuts, no man opens, the spirit of Jesus stopped them. [Acts 16:7] The open door may also be taken to mean the Scripture, through which we come in towards Christ with the steps of faith. So he opens the door with the key of David, and no man shuts, because no one is able, with any veil of the Jews, to cast a shadow in the elect's hearts over the words of the sacred Scripture, which he made his elect understand spiritually by the grace of his incarnation. On the other hand, he shuts it for the unbelieving Jews, and it cannot be opened by anyone. This too can be analyzed symbolically: because thou hast a little strength, and hast kept my word, and hast not denied my name. Here it is no small question that arises: how can someone have only a little strength if they keep the word of God and do not deny his name? We should know therefore that it was said according to the estimation of human weakness, because, while the elect are capable of many things through Christ, they presume that they are capable of little. Indeed Thou hast a little strength is said as if it were said "Thou thinkest that thou hast a little strength."”

“All the judgments of Scriptures refer to this one judgment; hence, He is called the Prince of Peace. Wherefore just rewards come forth from Him. He, then, "has the key of David, He who opens and no one shuts." And such a key is the mystery of union.”

“This is the Key of David which the Inspired Word is teaching, this Word, who is uncreated in the bosom of the Father, incarnate in the Virgin's womb, inspired in your heart by faith. He penetrates angelic and human minds, and coming into them, He makes them understand these visions, because He is pure understanding and spirit; but He does so in different degrees, for there are different depths in the penetration by which the soul is intimately led to Him.”

“And it was said to the Angel of Philadelphia, who was the sixth, "Thus says the holy one, the true one, he who has the key of David, he who opens and no one shuts, and who shuts and no one opens: I know thy works. Behold, I have caused a door to be opened before thee." And he said that now for the first time the understanding of Scripture would be given, either as revelation or as the key of David, to a single person or to a great number of people; and I believe rather that it would be given to a great number.”

“To those in Philadelphia, it says, write: These things says the Holy one, the True one. The Holy one is indeed the Son of God. Then this is also testified by the Seraphim, who unite a threefold "holy" into one lordship, having nothing earthly or sinful, even though the Word became flesh. "For he committed no sin. Nor was deceit found in his mouth," according to the voice of the prophet Isaiah. (Isa. 53:9) And it is true that it is also said, the True one; and He also is called God, and the title is not false. For God is truly Emmanuel, even if the accursed Nestorius does not wish this; He became man without ceasing to be God, and He is truly man, even if the God-hated Eutyches finds this difficult. Which is truly the case, and nothing but a matter of fact, not as the Nestorians claim by way of allegory, nor as the Eutychians assert by opinion or imagination, nor the accursed and detestable race of the Manichaeans. He says He who holds the key of David. To hold the key means to hold authority. For the one entrusted with the key of the house is given the authority to open and to shut. And this is clearly demonstrated in the Gospels in the promises made to Peter. "For I will give you the keys of the kingdom of heaven; and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven." (Matt. 16:19) Since therefore the key signifies authority, the one who holds the key of David clearly indicates that just as David reigned over the physical Israel, so too I reign over both the physical and the spiritual Israel, although the nature of the authority is exchanged with incomparable superiority. For what equality is there between man and God? This was also proclaimed by the divine angel Gabriel to the virgin concerning the Lord, saying, "And the Lord God will give him the throne of David his father; and he shall reign over the house of David forever, and of his kingdom there shall be no end." (Luke 1:32-33) Since Christ founded a likeness to the kingdom of David, it is fittingly said by the one who holds the key of David. Then, having remained firm in the manner of the key, He brings it forward: He who opens and no one will shut; and He who shuts, and no one opens. For if God is the one who justifies; who is the one who condemns? If God is the one who condemns; who is the one who justifies? For He says to open and to shut, means to justify and to condemn.”