1 John 4:13

In this we know that we abide in Him... by His Spirit, &c. By His Spirit, i.e., the participation of the Spirit, the communication of grace and charity, which are the gifts of the Spirit.

In the preceding verse S. John said that God abides in us, and consequently we in God by charity. For so loving He abides in the lover and the beloved. For so God loves us and we God. He here inculcates the same thing, repeats it, and as it were enforces it by a reason. The reason is this, He who hath the Spirit of God abides in God, and God in him: but he who hath charity hath the Spirit of God. Therefore he who hath charity abides in God and God in him. The major premiss is self-evident, because where the Spirit of God is, there is God Himself. But where God is, there He unites to Himself the subject in which He is, and by, as it were, the infinity of His Essence incorporates and absorbs it, so that the subject should be more in God than God in it. He therefore who hath experience in himself of the Spirit of God, i.e. of charity, this man feels God's presence and liberality. He feels God to be in him and himself in God, in such wise that God is bestowing His gifts upon him, and printing His perfect image in him, according to the words, "he that is joined to the Lord is one Spirit." (1 Cor. xii.) Ver. 14. And we have seen and do testify, &c. These words have reference to the 9th verse, where he saith that God hath shown His love to us by sending His Son. This he now proves and confirms by his own testimony, and that of the other Apostles. For they were the eye and ear witnesses, who saw, heard, and conversed with Christ Incarnate, as he said in the beginning of the Epistle.

This is an allusion to S. John's Gospel (Joh 3:17). "For God sent not His Son into the world to judge the world, but that through Him the world might be saved." Whence S. Bernard saith (de amor Dei, c. 8), "Christ Himself is our Love, by whom we attain to Thee, by whom we embrace Thee: for how otherwise, 0 incomprehensible Majesty, couldest Thou appear comprehensible to the soul that loveth Thee? For although no understanding of any soul or spirit can comprehend Thee, yet the love of the loving soul comprehends Thee wholly as thou art."