For when God made promise to Abraham, because he could swear by no greater, he sware by himself,

When God made His promise to Abraham, since He had no one greater to swear by, He swore by Himself,

Saying, Surely blessing I will bless thee, and multiplying I will multiply thee.

saying, “I will surely bless you and multiply your descendants.”

And so, after he had patiently endured, he obtained the promise.

And so Abraham, after waiting patiently, obtained the promise.

For men verily swear by the greater: and an oath for confirmation is to them an end of all strife.

Men swear by someone greater than themselves, and their oath serves as a confirmation to end all argument.

Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath:

So when God wanted to make the unchanging nature of His purpose very clear to the heirs of the promise, He guaranteed it with an oath.