1 John 4:13-15
Because He Has Given Us His Spirit
So how do we know that we abide in God and God in us? We have spoken of tests whereby we may examine ourselves: Believing and confessing Christ, doing good deeds, and loving our brothers. And these are good outward tests; I say “outward” because they are matters we may observe. But there is ultimately only one way to know for sure, and that is simply this: “Because he has given us of His Spirit.”
In a word, the Christian knows that he has been born again because God has sent His Spirit into him as seal and guarantee (1 Corinthians 1:22). Granted, this appears very subjective, but it is true. Salvation is the inward work of the Spirit upon the heart of the one the Spirit brings to saving faith. He comes to convict and convince (John 16:7-15) working inside the person through outward means such as the word, the testimony of other believers, and circumstances until that person finally surrenders. Only the spiritual man can understand spiritual things (1 Corinthians 2:14), but to do so he must be made into a spiritual man. This is the work of the Spirit who rebirths that one unto saving faith. And as the Spirit takes up residence and abides in that one born again, then that one has the testimony of the Holy Spirit within and may know and declare, “I am a child of God.”
The Holy Spirit is the greatest gift the Father gives to believers (Luke 11:13; Acts 2:33). Prior to our conversion, he convicts and convinces. Upon our rebirth through the Spirit and after, he sustains, empowers, gifts, sanctifies, nurtures, and most important for John—abides within us. Indeed, the Spirit is God in us, and it is because he resides in us that we believe.
And because the Spirit lives within us, we are able to affirm wholeheartedly, “We have seen and testify that the Father has sent His Son to be the Savior of the world.” But we were not there to see the Son. We were not there to see his miracles, his crucifixion, his resurrection and ascension. Even so, none of this matters. For we believe none of this on the basis of our senses; we believe the gospel on the basis of the work of the Spirit within us. No argument will persuade us otherwise, for the believer requires no other proof. And as we believe, so we confess that Jesus is the Son of God.
And this is ultimately how we know that we abide in Him, and He in us. Indeed, the three outward tests are to confirm the inward: That we have been born again of the Spirit who provides the inward witness.