Genesis 15:4, commentary and pictures

The word of the Lord declared that Abraham's trusted household servant would not be his heir, but that his heir would be a son coming forth out of his own body, so that his descendants would come through his own family. The servant thus set aside was Eliezer. Abraham had proposed to adopt Eliezer as heir to inherit the promise, having considered him as the one who would receive it.

The assurance answered Abraham's distress at having no child to inherit the promise. God confirmed the promise by directing Abraham to the stars and promising descendants as numerous as they, too numerous to count. Abraham at first accepted the promise of a son without question.

The promise required Abraham to trust God's power to give him a son even though he and Sarah were old, and God permitted a delay in fulfilling His word to test Abraham's faith to trust His power to fulfill it in His own time and way. God's promise did not depend on Abraham and Sarah devising their own way to fulfill it, but thinking a child impossible in old age they tried to fulfill the divine purpose by Sarah's plan of Abraham taking her handmaiden Hagar as a secondary wife, which showed a failure to endure the trial and to trust God. That resort to Hagar brought unhappiness and evil not only to Abraham's own household but to future generations, for although polygamy was then so widespread it had ceased to be regarded as sin, it was still a violation of God's law and fatal to the sacredness and peace of the family.