Augustine of Hippo

That is, it will be well, but if not, then after that thou shalt cut it down; namely, when Thou shalt come to judge the quick and the dead. In the mean time it is now spared.

Catena Aurea by Aquinas ↗

Gregory the Dialogist

And there are many who hear rebukes and yet scorn to return to penitence, and, unfruitful to God, stand green in this world. But let us hear what the vinedresser adds: "If indeed it bears fruit; but if not, you shall cut it down in the future." Because indeed one who does not wish to grow fat here toward fruitfulness through rebuke will fall there from where he can no longer rise through penitence; and he will be cut down in the future, although here he may seem to stand green without fruit.

Forty Gospel Homilies, Homily 31 ↗

Bede

And if it indeed produces fruit. But if not, in the future you will cut it down. When He said, "And if it indeed produces fruit," He did not add anything, but suspended the judgment. However, when He added, "But if not," He immediately attached the judgment of future condemnation, saying, "In the future you will cut it down," because evidently He saw the synagogue much more inclined to deny than to confess God. …

On the Gospel of Luke ↗