Since there are two commandments, the love of God and the love of our neighbour, on which hang the Law and the Prophets, not without reason does Scripture put one for both; sometimes the love of God; as in that, “We know that all things work together for good to them that love God;” [Rom 8:28] and sometimes the love of our neighbour; as in that, “All the law is fulfilled in one word, even in this, Thou shalt love thy neighbour as thyself.” [Gal 5:14]
And that because if a man love his neighbour, it follows therefrom that he loves God also; for it is the selfsame affection by which we love God, and by which we love our neighbour, save that we love God for Himself, but ourselves and our neighbour for God’s sake.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Trin., viii. 7