The Counsels and Purposes of God are Immutable

The counsel of the LORD stands forever, the purposes of His heart to all generations.

Many plans are in a man’s heart, but the purpose of the LORD will prevail.

Therefore the earth will mourn and the heavens above will grow dark. I have spoken, I have planned, and I will not relent or turn back.”

Yet before the twins were born or had done anything good or bad, in order that God’s plan of election might stand,

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.