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

Now he which stablisheth us with you in Christ, and hath anointed us, is God; Who hath also sealed us, and given the earnest of the Spirit in our hearts.

Now he that hath wrought us for the selfsame thing is God, who also hath given unto us the earnest of the Spirit.

In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.