Objection 1. It would seem that love does not cause mutual indwelling, so that the lover be in the beloved and vice versa. For that which is in another is contained in it. But the same cannot be container and contents. Therefore love cannot cause mutual indwelling, so that the lover be in the beloved and vice versa.
Objection 2. Further, nothing can penetrate within a whole, except by means of a division of the whole. But it is the function of the reason, not of the appetite where love resides, to divide things that are really united. Therefore mutual indwelling is not an effect of love.
Objection 3. Further, if love involves the lover being in the beloved and vice versa, it follows that the beloved is united to the lover, in the same way as the lover is united to the beloved. But the union itself is love, as stated above (Article 1). Therefore it follows that the lover is always loved by the object of his love; which is evidently false. Therefore mutual indwelling is not an effect of love.
On the contrary, 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.
I answer that, This effect of mutual indwelling may be understood as referring both to the apprehensive and to the appetitive power. Because, as to the apprehensive power, 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. Thus it is written concerning the Holy Ghost, Who is God's Love, that He "searcheth all things, yea the deep things of God" (1 Corinthians 2:10).
As the appetitive power, the object loved is said to be in the lover, inasmuch as it is in his affections, by a kind of complacency: causing him either to take pleasure in it, or in its good, when present; or, in the absence of the object loved, by his longing, to tend towards it with the love of concupiscence, or towards the good that he wills to the beloved, with the love of friendship: not indeed from any extrinsic cause (as when we desire one thing on account of another, or wish good to another on account of something else), but because the complacency in the beloved is rooted in the lover's heart. For this reason we speak of love as being "intimate"; and "of the bowels of charity." On the other hand, the lover is in the beloved, by the love of concupiscence and by the love of friendship, but not in the same way. For the love of concupiscence is not satisfied with any external or superficial possession or enjoyment of the beloved; but seeks to possess the beloved perfectly, by penetrating into his heart, as it were. Whereas, in the love of friendship, the lover is in the beloved, inasmuch as he reckons what is good or evil to his friend, as being so to himself; and his friend's will as his own, so that it seems as though he felt the good or suffered the evil in the person of his friend. Hence it is proper to friends "to desire the same things, and to grieve and rejoice at the same," as the Philosopher says (Ethic. ix, 3 and Rhet. ii, 4). Consequently in so far as he reckons what affects his friend as affecting himself, the lover seems to be in the beloved, as though he were become one with him: but in so far as, on the other hand, he wills and acts for his friend's sake as for his own sake, looking on his friend as identified with himself, thus the beloved is in the lover.
In yet a third way, mutual indwelling in the love of friendship can be understood in regard to reciprocal love: inasmuch as friends return love for love, and both desire and do good things for one another.
Reply to Objection 1. The beloved is contained in the lover, by being impressed on his heart and thus becoming the object of his complacency. On the other hand, the lover is contained in the beloved, inasmuch as the lover penetrates, so to speak, into the beloved. For nothing hinders a thing from being both container and contents in different ways: just as a genus is contained in its species, and vice versa.
Reply to Objection 2. The apprehension of the reason precedes the movement of love. Consequently, just as the reason divides, so does the movement of love penetrate into the beloved, as was explained above.
Reply to Objection 3. This argument is true of the third kind of mutual indwelling, which is not to be found in every kind of love.
Source: Summa Theologica (New Advent)