It is written (1 John 4:16): "He that abideth in charity abideth in God, and God in him." Now charity is the love of God. Therefore, for the same reason, every love makes the beloved to be in the lover, and vice versa... the beloved is said to be in the lover, inasmuch as the beloved abides in the apprehension of the lover, according to Philippians 1:7, "For that I have you in my heart": while the lover is said to be in the beloved, according to apprehension, inasmuch as the lover is not satisfied with a superficial apprehension of the beloved, but strives to gain an intimate knowledge of everything pertaining to the beloved, so as to penetrate into his very soul.

I-II, q. 28, art. 2 - Summa Theologica (1265–1274)