Charles Spurgeon Commentary Romans 8:15-16

"For ye received not the spirit of bondage again unto fear; but ye received the spirit of adoption, whereby we cry, Abba, Father. The Spirit himself beareth witness with our spirit, that we are children of God:" — Romans 8:15-16 (ASV)

For you have not received the spirit of bondage again to fear; but you have received the Spirit of adoption, whereby we cry, Abba, Father. The Spirit itself bears witness with our spirit, that we are the children of God.

That is, when we have the Spirit, when we are renewed in the spirit of our minds, when we come into the domain of the Spirit, and quit the tyranny of the flesh. Then the Spirit bears witness with our spirit that we are the children of God.