John Chrysostom

How is it, then, that in his prayer he says: "If it be possible?" He is showing the weakness that belongs to a human nature. Human nature would prefer not to be torn from the present life. It would draw back and shrink from death. Why? Because God has implanted in human nature a love for the life of this world.

On The Incomprehensible Nature Of God 7.46 ↗

Augustine of Hippo

He said not, if He could do it, but if it could be done; for whatever He wills is possible. We must therefore understand, if it be possible, as if it were; if He is willing. And lest any one should suppose that He lessened His Father's power, he shows in what sense the words are to be understood; for there follows, And he said, Abba, Father, all things are possible unto thee. By which He sufficiently shows, that the words, if it be possible, must be understood not of any impossibility, but of the will of His Father. …

Catena Aurea by Aquinas ↗

Ambrose of Milan

He has fully taken upon himself the flesh of humanity, and with it human affections. So you read in Scripture that "going a little farther, he fell on the ground and prayed that, if it were possible, the hour might pass from him." Here he speaks not in the voice of God but as fully human. For how could God be ignorant of the possibility or impossibility of anything? Or is anything beyond God's ability, when as Scripture itself says: "For you nothing is impossible?"

Exposition of the Christian Faith 2.5.42 ↗

Bede

And he prayed that if it was possible the hour would pass from him, and said: Abba, etc. What was that voice, except the sound of our weakness? Many still frail ones grieve at the future death; but let them have a right heart, let them avoid death as much as they can; but if they cannot, let them say what the Lord himself, not for himself, but for us said. For what did he say? Father, if it is possible, let this cup pass from me. Behold you have the human will expressed. Now see the right heart.

On the Gospel of Mark ↗