John Calvin Commentary Genesis 21:24

"And Abraham said, I will swear." — Genesis 21:24 (ASV)

And Abraham said, I will swear. Although he had the stronger claim of right, he nevertheless refuses nothing which belonged to the duty of a good and moderate man.

And truly, since it is fitting for the sons of God to be freely ready for every duty, nothing is more absurd than for them to appear reluctant and morose when what is just is required of them. He did not refuse to swear because he knew it was lawful for covenants to be ratified between men in the sacred name of God. In short, we see Abraham willingly submitting himself to the laws of his vocation.