What made God say that Abraham will be the father of the nation?
God promised Abraham that he would become the father of many nations. See: How was Abraham famous?
God didn't have to offer that promise. He chose to make that promise and through that promise we see God's power.
"And we desire that each one of you show the same diligence so as to realize the full assurance of hope until the end, so that you will not be sluggish, but imitators of those who through faith and patience inherit the promises. 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 unchangeable things in which it is impossible for God to lie, we who have taken refuge would have strong encouragement to take hold of the hope set before us. This hope we have as an anchor of the soul, a hope both sure and steadfast and one which enters within the veil" (Hebrews 6:11-19).