The Spirit Himself bears witness with our spirit that we are children of God,
Romans 8:16
Surrounding context
8:15 For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption by whom we cry out, "Abba, Father."
8:16 The Spirit Himself bears witness with our spirit that we are children of God,
8:17 and if children, then heirs--heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together.