John Calvin Commentary Galatians 3:15

"Brethren, I speak after the manner of men: Though it be but a man`s covenant, yet when it hath been confirmed, no one maketh it void, or addeth thereto." — Galatians 3:15 (ASV)

I speak after the manner of men. By this expression, he intended to make them ashamed. It is highly disgraceful and shameful that the testimony of God should have less weight with us than that of a mortal man. In demanding that the sacred covenant of God should receive no less respect than is commonly given to ordinary human transactions, he does not place God on a level with men. The immense distance between God and men is still left for their consideration.

Though it be but a man’s covenant. This is an argument from the less to the greater. Human contracts are universally admitted to be binding: how much more what God has established? The Greek word διαθήκη, used here, signifies more frequently, what the Latin versions here render it, (testamentum), a testament; but it also sometimes means a covenant, although for this latter meaning, the plural form is more commonly used.

It is of little importance for this passage whether you explain it as covenant or testament. The situation is different with the Epistle to the Hebrews, where the apostle unquestionably alludes to testaments (Hebrews 9:16, 17); but here I prefer to understand it simply as the covenant which God made.

The analogy from which the apostle argues would not apply as strictly to a testament as to a covenant. The apostle appears to reason from human agreements to that solemn covenant into which God entered with Abraham. If human agreements are so firm that nothing can be added to them, how much more must this covenant remain inviolable?