Augustine of Hippo : "In this we know that we dwell in Him and He in us, because He hath given us of His Spirit." It is well: thanks be to God! We come to know that He dwelleth in us. And whence come we to know this very thing, to wit, that we do know that He dwelleth in us? Because John himself has said this: "Because He hath given us of His Spirit." Whence know we that He hath given us of His Spirit? This very thing, that He hath given thee of His Spirit, whence comest thou to know it? Ask thine own bowels: if they are full of charity, thou hast the Spirit of God. Whence know we that by this thou knowest that the Spirit of God dwelleth in thee? "Because the love of God is shed abroad in our hearts by the Holy Spirit which is given unto us." — Ten Homilies on 1 John 8 Bede : In this we know that we abide in Him, etc. This very thing, because He gave His Spirit to you, how do you know? Ask your own inner parts. If they are full of charity, you have the Spirit of God, as Paul attests, who says: Because the charity of God is poured out in our hearts by the Holy Spirit who is given to us (Rom. V). — Commentary on the Catholic Epistles Oecumenius : By this we know that we abide in Him and He in us, because He has given us of His Spirit. And we have seen and testify that the Father has sent His Son as the Savior of the world. For this is effective and has no defect, when it is pure. This is the sign that we abide in Him and He in us, which He has also given us through His Spirit. For He who is pure grants us purity and is perfect from all filth. Therefore, since we communicate with Him through pure love, we also, He says, who have seen Him according to the flesh, have known Him, and we testify that the Father sent Him as the Savior of the world. But besides our own knowledge, He also instructed us, leading us more perfectly into this understanding, when He said: "I came forth from the Father and have come into the world." (Jn. 16:28) This pertains to the fact that He sent His Only Begotten Son into the world for our love. Again, He made it clearer to others: "For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish." (Jn. 3:16) And: "I did not come to judge the world, but to save the world." (Jn. 12:47) Therefore, we have, from our own sight, He says, and from the teaching of the Only Begotten who is in the bosom of the Father (Jn. 1:18), as it is said in the Gospels, and from the operation through mutual love, that God is in us, and He has given us of His Spirit, and we participate with Him. — Commentary on 1 John