5 Now see that to act against love is to act against God. Let no man say, “I sin against man when I do not love my brother, (mark it!) and sin against man is a thing to be taken easily; only let me not sin against God.” How do you not sin against God, when you sin against love? “Love is God.” Do “we” say this? If we said, “Love is God,” haply some one of you might be offended and say, What has he said? What meant he to say, that “Love is God”? God “gave” love, as a gift God bestowed love. “Love is of God: Love IS God.” Look, here have ye, brethren, the Scriptures of God: this epistle is canonical; throughout all nations it is recited, it is held by the authority of the whole earth, it has edified the whole earth. You are here told by the Spirit of God, “Love is God.” Now if you dare, go against God, and refuse to love your brother!
Source: Homilies on the First Epistle of John (New Advent)