John Calvin confirms in his Tracts that the Spirit is the witness, seal and earnest of the believer’s adoption. Scripture calls the Holy Spirit, the Spirit of adoption because…
“he is the witness to us of the free benevolence of the God with which God the Father has embraced us in his beloved only-begotten Son to become a Father to us.”
This witness of the Spirit of the believer’s adoption is a co- witness. This co-witness takes place when the Spirit as Calvin writes,
“testifies to us, that we are the children of God, he at the same time pours into our hearts such confidence, that we venture to call God our Father.”
The ensuing privilege of the child of God is then assurance. This fruit is really bound up closely with the preceding privilege of adoption, that the believer has the Spirit as his witness. They are almost inseparable because as the Spirit witnesses to the heart, the believer is assured that he is a child of God. Calvin ties these two concepts together in his comments on Romans 8:16 where he says that,
“the Spirit of God gives us such a testimony, that when he is our guide and teacher, our spirit is made assured of the adoption of God: for our mind of its own self, without the preceding testimony of the Spirit, could not convey to us this assurance.”
This is corroborated in the statement that this assurance issues forth in a cry to God. While adoption affords assurance of God’s electing grace in the life of the believer, the Spirit works that assurance in the heart of God’s adopted children.