John Calvin Commentary Deuteronomy 27:26

"Cursed be he that confirmeth not the words of this law to do them. And all the people shall say, Amen." — Deuteronomy 27:26 (ASV)

Cursed is he that does not confirm. Although it was God’s purpose to summon the consciences of all people before Him, and, so that they might not only fear human judgments, He deliberately threatened them with the punishment of secret sins, yet the conclusion, which is now added, extends the same judgment to all iniquities of whatever kind. Indeed, He briefly declares that whoever does not perform what the Law requires is accursed.

From this Paul rightly infers that as many as are of the works of the Law are under the curse (Galatians 3:10). For let the most perfect person come forward; although they may have striven ever so diligently to keep the Law, they will have at least offended in some point or other. This is because the declaration of James must be kept in mind: Whoever shall keep the whole law, and yet offend in one point, is guilty of all; for He who forbade murder and adultery also forbade theft (James 2:10–11). Paul indeed does not quote the very words of Moses, for he thus cites his testimony:

Cursed is everyone that does not continue in all things which are written in the book of the law to do them (Galatians 3:10).

But there is no difference in the sense, since all are here condemned without exception who have not confirmed the Law of God, so as to fulfill completely whatever it contains. Therefore, it is clear that in whatever respect a deficiency reveals itself, it brings people under the curse. To this the Israelites are commanded to assent, so as to acknowledge that they were all, without exception, lost, since they were involved in the curse.

And nowadays, it is also necessary that every one of us should be struck with the same despair, so that, by embracing the grace of Christ, we should be delivered from this melancholy state of guilt, since He was made accursed for us, that He might redeem us from the curse of the Law (Galatians 3:13).