The Spirit itself beareth witness with our spirit, that we are the children of God:
Romans 8:16
King James Version (public domain)
Traditionally attributed to Paul · 1st century AD · Thirteen epistles of the New Testament are traditionally attributed to him
Context
14For as many as are led by the Spirit of God, they are the sons of God.
15For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
16The Spirit itself beareth witness with our spirit, that we are the children of God:
17And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.
18For I reckon that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us.