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” (Jn 15:2). Considering the literal sense, we see that a natural vine with branches that have many shoots bears less fruit, because the sap is spread out through all the shoots. Thus the vinedresser prunes away the extra shoots so that the vine can bear more fruit. It is the same with us. 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). And he does this so “that it may bear more fruit” (Jn 15:2), that is, grow in virtue, so that the more pruned or cleansed the more fruitful one is.
Saint Thomas Aquinas