After Abraham's separation from Lot the Lord said to him, “Lift up now thine eyes, and look from the place where thou art northward, and southward, and eastward, and westward: for all the land which thou seest, to thee will I give it, and to thy seed for ever. And I will make thy seed as the dust of the earth: so that if a man can number the dust of the earth, then shall thy seed also be numbered.” “The word of the Lord came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, and thy exceeding great reward.... And Abram said, Behold, to me Thou hast given no seed: and, lo, one born in my house is mine heir.” SR 76.2
As Abraham had no son, he at first thought that his trusty servant, Eliezer, should become his son by adoption, and his heir. But God informed Abraham that his servant should not be his son and heir, but that he should really have a son. “And He brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and He said unto him, So shall thy seed be.” SR 77.1
If Abraham and Sarah had waited in confiding faith for the fulfillment of the promise that they should have a son, much unhappiness would have been avoided. They believed that it would be just as God had promised, but could not believe that Sarah in her old age would have a son. Sarah suggested a plan whereby she thought the promise of God could be fulfilled. She entreated Abraham to take Hagar as his wife. In this they both lacked faith and a perfect trust in the power of God. By hearkening to the voice of Sarah and taking Hagar as his wife Abraham failed to endure the test of his faith in God's unlimited power, and brought upon himself and upon Sarah much unhappiness. The Lord intended to prove the firm faith and reliance of Abraham upon the promises He had made him. SR 77.2