John Chrysostom

The wonder of the disciples was the consequence of imperfect faith, for this was no great thing for God to do; since then they did not clearly know His power, their ignorance made them break out into wonder; and therefore it is added, And Jesus answering saith unto them, Have faith in God. For verily I say unto you, That whosoever shall say unto this mountain, &c. That is; Thou shalt not only be able to dry up a tree, but also to change a mountain by thy command and order.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

Although Matthew says that the fig tree withered at once and that the disciples, seeing this, marveled, do not be troubled hearing now from Mark that they saw the withered fig tree only on the next day. What Matthew said should be understood thus: "And the fig tree withered at once" — stop here; then read: "Seeing this, the disciples marveled." When did they see it? — not at once, but on the next day. Whoever understands it this way will encounter no difficulty.

Commentary on Mark ↗

Augustine of Hippo

The meaning is not that it dried up at the time, when they saw it, but immediately after the word of the Lord; for they saw it, not beginning to dry up, but completely dried up; and they thus understood that it had withered immediately after our Lord spoke.

Catena Aurea by Aquinas ↗

Bede

And as they passed by in the morning, they saw the fig tree dried up from the roots. Not only the branches or the trunks of the barren fig tree, but even the root itself, drying up, showed the sentence of divine reprobation upon it. And John says that the axe is laid to the root of the trees (Matt. III). …

On the Gospel of Mark ↗