"The Spirit Himself testifies with our spirit that we are children of God,"
God's children are those who have been born again by the Spirit of God. They know who they are because the Holy Spirit testifies to their spirit that this is true. He speaks right to their heart and they are confident that they belong to God. Jewish law stated everything had to be established by the testimony of two or three witnesses (Deuteronomy 17:6). There are two witnesses to our salvation: our own witness and the witness of the Holy Spirit. "...the Holy Spirit bears witness with our spirit when we cry, "Jesus is Lord!" But that is not the only cry the Spirit prompts in our hearts. Another is, "Abba! Father!" In other words, the Spirit produces two profound changes in us toward God: One is a humble demeanor of submission: Jesus, the Son of God, is my Lord, my Master; I am His subject; He is my Ruler, my Sovereign. And the other is the joyful, bold, childlike demeanor of confidence: God is my Father." -- John Piper