"For when God made the promise to Abraham,
since He could swear by no one greater,
He swore by Himself, saying,
'I will surely bless you
and I will surely multiply you."
And so having patiently waited, he obtained the promise.
For men swear by one greater than themselves,
and with them an oath given as confirmation
is an end of every dispute.
In the same way God, desiring even more
to show to the heirs of the promise
the unchangeableness of His purpose,
interposed with an oath,
so that by two interchangeable things in which
I cringe whenever I hear God's name in vain.
Some neighbor kids had a hard time restraining themselves
at my house so I finally told them, from now on, every time you
use God's name in vain, you have to drop and give me ten.
Funny thing. They quit swearing.
When God swears, we better listen!
"'When God made the promise to Abraham' The promise referred to is that made to Abraham when he had offered his son Isaac on the altar, Genesis 22:16-18: 'By myself have I sworn, saith the Lord; for because thou hast done this thing, and hast not withheld thy son, thy only son; that in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the seashore; and thy seed shall possess the gate of his enemies; and in thy seed shall all the nations of the earth be blessed."
God certainly multiplied Abraham's descendants. He could overwhelm ancestry.com with all his kinfolk. But in the last part of that quote, when it says 'in thy seed shall all the nations of the earth be blessed,' it means through Christ, not only were the Jews blessing proclaimed, but also the Gentiles. We are heirs of the promise.
So why did God swear? Because He can if He wants to. Andrew Murray, "He had already spoken of God's oath in His wrath, 'They shall not enter into My rest...Here he wishes to show believers what strong encouragement they have in God's oath to expect most confidently the fulfilment of the promise...The oath of God plainly proves that the thing He seeks above everything is--faith; He wishes to be trusted. Faith is nothing but depending on God to do for us what we cannot do--what He has undertaken to do...He is ready, He longs, as God, Himself to work in us all that He has promised. He cannot do this except as we open our hearts to Him, and yield ourselves in stillness and surrender for Him to do His work. Until this faith takes possession of us, we are always seeking to do His work, and we hinder Him. Faith teaches us in deep humility and dependence, in meekness and patience, to place ourselves in God's hands, to make way for Him, and to wait His time.'
For example, we have a son, not that we are planning to sacrifice him like Abraham, but we are trying to help him find his place in the world. He'll be graduated from his group home next month and needs his own apartment. God has already miraculously provided SSI. So now we were looking all over our area at fifteen to twenty apartments, and everything was either a year or two waiting list or was way over budget. Of course we had been and were constantly praying as we searched. Finally, we found one. One is all we need. It is at the right price. We accept it as God's choice. We take it as His blessing.
Adam Clarke says, "If God had sworn by anything finite, that thing might fail, and then the obligation would be at an end, but He has sworn by what is infinite, and cannot fail; therefore His oath is of eternal obligation."
God gives a promise and an oath, to show that it is impossible for God to lie. "His unchangeable purpose, to call the Gentiles to salvation by Jesus Christ...and finally to bring every persevering believer, whether Jew or Gentile, to eternal glory...God invites thee! Jesus spreads His hands to receive thee! God hath sworn that He willeth not the death of a sinner...take His promise, credit what He hath spoken and sworn! Take encouragement! Believe on the Son of God, and thou shalt not perish, but have everlasting life." (Adam Clarke)