Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
1st John 3:1

For as many as are led by the Spirit of God, they are the sons of God.
For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
The Spirit itself beareth witness with our spirit, that we are the children of God
Romans 8:14-16