John Calvin Commentary Genesis 26:31

"And they rose up betimes in the morning, and sware one to another. And Isaac sent them away, and they departed from him in peace." — Genesis 26:31 (ASV)

And sware one to another. Isaac does not hesitate to swear; partly so that the Philistines may be more easily appeased, and partly so that he may not be suspected by them. And this is the legitimate method of swearing, when men mutually bind themselves to the cultivation of peace. A simple promise, indeed, should have sufficed; but since deceptions or inconstancy cause people to distrust each other, the Lord grants them the use of His name, so that this more holy confirmation may be added to our covenants; and He not only permits, He even commands us to swear whenever it is necessary (Deuteronomy 6:13). Meanwhile, we must beware lest His name be profaned by rash swearing.