Romans 8:16

The Spirit himself doth testify with our spirit, that we are children of God;

2 Corinthians 1:22

who also sealed us, and gave the earnest of the Spirit in our hearts.

2 Corinthians 5:5

And He who did work us to this self-same thing is God, who also did give to us the earnest of the Spirit;

Ephesians 1:13

in whom ye also, having heard the word of the truth -- the good news of your salvation -- in whom also having believed, ye were sealed with the Holy Spirit of the promise,

2 Corinthians 1:12

For our glorying is this: the testimony of our conscience, that in simplicity and sincerity of God, not in fleshly wisdom, but in the grace of God, we did conduct ourselves in the world, and more abundantly toward you;

Ephesians 4:30

and make not sorrowful the Holy Spirit of God, in which ye were sealed to a day of redemption.

1 John 3:19-22

and in this we know that of the truth we are, and before Him we shall assure our hearts,

1 John 4:13

in this we know that in Him we do remain, and He in us, because of His Spirit He hath given us.

1 John 5:10

He who is believing in the Son of God, hath the testimony in himself; he who is not believing God, a liar hath made Him, because he hath not believed in the testimony that God hath testified concerning His Son;

Romans 8:23

And not only so, but also we ourselves, having the first-fruit of the Spirit, we also ourselves in ourselves do groan, adoption expecting -- the redemption of our body;

Romans 8:26

And, in like manner also, the Spirit doth help our weaknesses; for, what we may pray for, as it behoveth us, we have not known, but the Spirit himself doth make intercession for us with groanings unutterable,

Treasury of Scripture Knowledge

Summary

The Spirit itself beareth witness with our spirit, that we are the children of God:

General references

Bible References

Spirit

Romans 8:23
And not only so, but also we ourselves, having the first-fruit of the Spirit, we also ourselves in ourselves do groan, adoption expecting -- the redemption of our body;
2 Corinthians 1:22
who also sealed us, and gave the earnest of the Spirit in our hearts.
2 Corinthians 5:5
And He who did work us to this self-same thing is God, who also did give to us the earnest of the Spirit;
Ephesians 1:13
in whom ye also, having heard the word of the truth -- the good news of your salvation -- in whom also having believed, ye were sealed with the Holy Spirit of the promise,
Ephesians 4:30
and make not sorrowful the Holy Spirit of God, in which ye were sealed to a day of redemption.
1 John 4:13
in this we know that in Him we do remain, and He in us, because of His Spirit He hath given us.

With our

2 Corinthians 1:12
For our glorying is this: the testimony of our conscience, that in simplicity and sincerity of God, not in fleshly wisdom, but in the grace of God, we did conduct ourselves in the world, and more abundantly toward you;
1 John 3:19
and in this we know that of the truth we are, and before Him we shall assure our hearts,
1 John 5:10
He who is believing in the Son of God, hath the testimony in himself; he who is not believing God, a liar hath made Him, because he hath not believed in the testimony that God hath testified concerning His Son;

General references

Galatians 3:26
for ye are all sons of God through the faith in Christ Jesus,