John Calvin Commentary Galatians 3:26

"For ye are all sons of God, through faith, in Christ Jesus." — Galatians 3:26 (ASV)

For you are all the children of God. It would be unjust, and in the highest degree unreasonable, that the law should hold believers in perpetual slavery. This is proved by the additional argument, that they are the children of God. It would not be enough to say that we are no longer children, unless it were added that we are freemen; for in slaves age makes no alteration. The fact of their being the children of God proves their freedom. How? By faith in Christ Jesus; for

as many as received him, to them gave he power to become the sons of God, even to them that believe on his name.
(John 1:12)

Since, then, by faith we have obtained adoption, by faith likewise we have obtained our freedom.