The interpretation timeline
Psalm 93:18 Douay-Rheims

If I said: My foot is moved: thy mercy, O Lord, assisted me.

A.D. 430 – 1849 3 witnesses 6 excerpts

Patristic

before A.D. 750
430
A.D.
Augustine of Hippo Patristic
A.D. 354–430
“And if in labor we faint, let us implore help. He helps the one fighting, who started the contest. For God does not watch you fighting as the people watch the charioteer; he does not want to shout, he does not know how to help. God does not watch you fighting as the trainer watches the athlete; he prepares a wreath of straw, but does not know how to supply strength to the laborer, nor can he; for he is a man, not God. And perhaps while he watches, he labors more by sitting than the other by wrestling. For God, when he watches his fighters, helps those who are called upon. For the voice of his athlete is in the Psalm: "If I said: My foot is moved, your mercy, Lord, helped me." Therefore let us not be lazy, my brothers, let us ask, seek, knock. "For everyone who asks receives, and he who seeks finds, and to him who knocks it will be opened."”
Source
“However, do not think that you will always or immediately be strong, nor completely despair. That alternation of weakness and strength in the hands of God's servant Moses might have been your alternation. Sometimes you falter in temptations, but you do not succumb. He would drop his hands for a little while, but he did not fall completely. "If I said: My foot slipped; Your mercy, O Lord, supported me." Therefore, do not fear: a helper is present on the journey, the one who was not absent in Egypt as a liberator. Do not fear, undertake the way, proceed confidently. Sometimes he lowered his hands, sometimes he raised them; yet Amalek was defeated. He could rebel, yet he could not overcome.”
Source
“Therefore, to conclude our discussion from where we began, let us pray, and presume upon God: let us live as He commands, and where we falter in this life, let us call upon Him, just as the disciples called upon Him saying: "Lord, increase our faith." And Peter presumed, and faltered: yet he was not despised and drowned, but was uplifted and raised up. Indeed, from where did he presume? It was not from his own strength: it was from the Lord. How? "Lord, if it is You, command me to come to You on the water." For the Lord was walking on the water. "If it is You, command me to come to You on the water. For I know that if it is You, You command and it is done." And He said: "Come." He descended at His command, and trembled in his own weakness. Yet when he trembled, he cried out to Him: "Lord," he said, "save me." Then the Lord reached out His hand to him, and said: "O you of little faith, why did you doubt?" He Himself invited, He liberated the one who was faltering and trembling; so that it might be fulfilled, what is said in the Psalm: "If I said, My foot slips; Your mercy, O Lord, supported me."”
Source
“"If I said, My foot has slipt; Your mercy, O Lord, held me up" [Psalm 94:18]. See how God loves confession. Your foot has slipt, and you say not, my foot has slipt; but you say you are firm, when you are slipping. The moment you begin to slip or waver, confess thou that slip, that you may not bewail your total fall; that He may help, so that your soul be not in hell. God loves confession, loves humility. You have slipped, as a man; God helps you, nevertheless: yet say, "My foot has slipt." Why do you slip, and yet sayest, I am firm? "When I said, My foot has slipt, Your mercy, O Lord, has held me up." Just as Peter presumed, but not in strength of his own. The Lord was seen to walk upon the sea, trampling on the heads of all the proud in this life. In walking upon the foaming waves, He figured His own course when He tramples on the heads of the proud. The Church too does trample upon them: for Peter is the Church Herself. Nevertheless, Peter dared not by himself walk upon the waters; but what said he? "Lord, if it be Thou, bid me come unto You on the water." [Matthew 14:28] He in His own power, Peter by His order; "bid me," he says, "come unto You." He answered, "Come." For the Church also tramples on the heads of the proud; but since it is the Church, and has human weakness, that these words might be fulfilled, "If I said, My foot has slipt," Peter tottered on the sea, and cried out, "Lord, save me!" [Matthew 14:30] and so what is here put, "If I said, My foot has slipt," is put there, "Lord, I perish." And what is here, "Your mercy, O Lord, has held me up," is there put, "And immediately Jesus stretched forth His hand, saying, O thou of little faith, wherefore did you doubt?" [Matthew 14:31] It is wonderful how God proves men: our very dangers render Him who rescues us sweeter unto us. For see what follows: because he said, "If I said, My foot has slipt, Your mercy, O Lord, has held me up." The Lord has become especially sweet unto him, in rescuing him from danger; and thus speaking of this very sweetness of the Lord, he exclaims and says, "O Lord, in the multitude of the sorrows that I had in my heart, Your comforts have refreshed my soul" [Psalm 94:19]. Many sorrows, but many consolations: bitter wounds, and sweet remedies.”
Source

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“When I said, my foot slippeth,.... There is no ground for me to stand upon; all is over with me; there is no hope nor help for me; I am just falling into ruin and destruction: such will be the desperate case of the church, at the time before referred to: thy mercy, O Lord, held me up; the extremity of his people is the Lord's opportunity; then is his set time to arise, and have mercy on them; then mercy steps in, lays a solid ground and foundation for hope, and holds up in its arms a sinking people, and revives a dying cause.”
Source
1849
A.D.
1774–1849
“Me. I was no sooner in danger, than I was relieved. (Calmet) — Feeding and complaining of my misery, I obtained thy aid. (Worthington)”
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.