From one and the same love, we love God and our neighbour; but God for His own sake, our neighbour for God’s. So that, there being two precepts of love, on which hang all the Law and the Prophets, to love God, and to love our neighbour, Scripture often unites them into one precept. For if a man love God, it follows that he does what God commands, and if so, that he loves his neighbour, God having commanded this. Wherefore He proceeds: Ye are My friends, if ye do whatsoever I command you.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: viii. de Trin. c. viii