The Spirit itself beareth witness with our spirit, that we are the children of God:
There is a spiritual instinct in the newborn believer that they are a son of God, the Holy Spirit tells them that it is so. The Spirit Himself bears witness with the believer’s spirit that they are a member of God’s family. The Spirit does it primarily though the Word of God. As a Christian reads the Bible, the Spirit confirms the truth that, because they have trusted the Saviour, they are now a child of God.