John Calvin Commentary Galatians 4:5

"that he might redeem them that were under the law, that we might receive the adoption of sons." — Galatians 4:5 (ASV)

That we might receive the adoption. The fathers under the Old Testament were certain of their adoption, but did not yet so fully enjoy their privilege. Adoption, like the phrase the redemption of our body (Romans 8:23), here signifies actual possession. As, at the last day, we receive the fruit of our redemption, so now we receive the fruit of adoption, of which the holy fathers did not partake before the coming of Christ; and therefore, those who now burden the church with an excess of ceremonies defraud her of the just right of adoption.