12 Let them see how many things, and what important things, are of no avail, if a certain single thing be wanting, and let them see what that one thing is. And herein let them hear not my words, but those of the apostle: "Though I speak with the tongues of men and of angels, and have not charity, I have become as sounding brass, or a tinkling cymbal. And though I have the gift of prophecy, and understand all mysteries, and all knowledge; and though I have all faith, so that I could remove mountains, and have not charity, I am nothing." What does it profit them, therefore, if they have both the voice of angels in the sacred mysteries, and the gift of prophecy, as had Caiaphas and Saul, that so they may be found prophesying, of whom Holy Scripture testifies that they were worthy of condemnation?
If they not only know, but even possess the sacraments, as Simon Magus did; if they have faith, as the devils confessed Christ (for we must not suppose that they did not believe when they said, "What have we to do with You, O Son of God? We know You who You are"; if they distribute of themselves their own substance to the poor, as many do, not only in the Catholic Church, but in the different heretical bodies; if, under the pressure of any persecution, they give their bodies with us to be burned for the faith which they like us confess: yet because they do all these things apart from the Church, not "forbearing one another in love," nor "endeavoring to keep the unity of the spirit in the bond of peace," insomuch as they have not charity, they cannot attain to eternal salvation, even with all those good things which profit them not.
Source: On Baptism, Against the Donatists (New Advent)