For if you live according to the flesh you will die; but if by the Spirit you put to death the deeds of the body, you will live. For as many as are led by the Spirit of God, these are sons of God. Romans 8:13–14

The Spirit Himself bears witness with our spirit that we are children of God. Romans 8:16

Behold what manner of love the Father has bestowed on us, that we should be called children of God! Therefore the world does not know us, because it did not know Him. 1 John 3:1

In this the children of God and the children of the devil are manifest: whoever does not practise righteousness is not of God, nor is he who does not love his brother. 1 John 3:10