6 In what sense then was it said a while ago, “Love is of God;” and now, “Love IS God?” For God is Father and Son and Holy Ghost: the Son, God of God, the Holy Ghost, God of God; and these three, one God, not three Gods. If the Son be God, and the Holy Ghost God, and that person loves in whom dwells the Holy Ghost: therefore “Love is God;” but “IS God,” because “Of God.” For you have both in the epistle; both, “Love is of God,” and, “Love is God.” Of the Father alone the Scripture has it not to say, that He is “of God:” but when you hear that expression, “Of God,” either the Son is meant, or the Holy Ghost.
Because while the apostle says, “The love of God is shed abroad in our hearts by the Holy Spirit which is given unto us:” let us understand that He who subsists in love is the Holy Ghost. For it is even this Holy Spirit, whom the bad cannot receive, even He is that Fountain of which the Scripture says, “Let the fountain of your water be your own, and let no stranger partake with you.” For all who love not God, are strangers, are antichrists. And though they come to the churches, they cannot be numbered among the children of God; not to them belongs that Fountain of life.
To have baptism is possible even for a bad man; to have prophecy is possible even for a bad man. We find that king Saul had prophecy: he was persecuting holy David, yet was he filled with the spirit of prophecy, and began to prophesy. To receive the sacrament of the body and blood of the Lord is possible even for a bad man: for of such it is said, “He that eats and drinks unworthily, eats and drinks judgment to himself.” To have the name of Christ is possible even for a bad man; i.e. even a bad man can be called a Christian: as they of whom it is said, “They polluted the name of their God.” I say, to have all these sacraments is possible even for a bad man; but to have charity, and to be a bad man, is not possible.
This then is the peculiar gift, this the “Fountain” that is singly one's “own.” To drink of this the Spirit of God exhorts you, to drink of Himself the Spirit of God exhorts you.
Source: Homilies on the First Epistle of John (New Advent)