Thomas_Aquinas_in_Stained_GlassI am the true vine, and my Father is the vinedresser. Every branch of mine that bears no fruit, he takes away, and every branch that does bear fruit he prunes, that it may bear more fruit.  You are already made clean by the word which I have spoken to you (John 15:1-3)

Just like a vine, although it seems to be of small account, nevertheless surpasses all trees in the sweetness of its fruit, so Christ, although he seemed to be despised by the world because he was poor, and seemed of small account and was publicly disgraced, nevertheless produced the sweetest fruit: “His fruit was sweet to my taste” (Song 2:3).

And so Christ is a vine producing a wine which interiorly intoxicates us: a wine of sorrow for sin: “You have given us to drink the wine of sorrow” (Ps 60:3); and a wine which strengthens us, that is, which restores us: “My blood is drink indeed” (6:55).

[…] He says, and my Father is the vinedresser….God cultivates us to make us better by his work, since he roots out the evil seeds in our hearts. As Augustine says, he opens our hearts with the plow of his words, plants the seeds of the commandments, and harvests the fruit of devotion.

[…] His interest in the good branches is to help them so they can bear more fruit. So he says, and every branch that does bear fruit he prunes, that it may bear more fruit.… For if we are well‑disposed and united to God, yet scatter our love over many things, our virtue becomes weak and we become less able to do good.

This is why God, in order that we may bear fruit, will frequently remove such obstacles and prune us by sending troubles and temptations, which make us stronger. Accordingly, he says, he prunes, even though one may be clean, for in this life no one is so clean that he does not need to be cleansed more and more: “If we say we have no sin, we deceive ourselves, and the truth is not in us” (1 Jn 1:8).

[…] He says, you are already made clean. It is like saying: I have said certain things about branches; and you are branches ready to be pruned so as to bear fruit. And you are clean by the word which I have spoken to you. The word of Christ, in the first place, cleanses us from error by teaching us: “He must hold firm to the sure word as taught, so that he may be able to give instruction in sound doctrine” (Titus 1:9).

[…] The word of Christ cleanses our hearts from earthly affections by inflaming them toward heavenly things. For the word of God by its power moves our hearts, weighed down by earthly things, and sets them on fire: “Is not my word fire?” (Jer 23:29)…. The word of Christ cleanses by the power of faith: God “cleansed their hearts by faith” (Acts 15:9).

Thomas Aquinas (1225-1274): Commentary on John, cap. 15, lect. 1, 1979-1988.