Abraham didn’t walk by faith to obtain the promised son until it became clear that walking by sight with his own plans guiding him just wouldn’t accomplish the purposes of God. God had promised Abraham and Sarah a son, and they had waited a long time for the fulfillment of that promise. As time went by, it seemed nothing was ever going to happen, and both of them grew impatient. This inspired Sarah to think of a plan: Abraham could lay with Hagar the maidservant and receive the promised son that way. In Genesis 16:2 we read that Abraham “heeded the voice of Sarai,” listening to the voice of his wife rather than the voice of God. He lay with Hagar and, sure enough, she bore the son Ishmael. But this was a plan of the flesh to bring about the plan of God—and that will never work.
Later on in Genesis 17:18–19 it says, “And Abraham said to God, ‘Oh, that Ishmael might live before You!’ Then God said: ‘No, Sarah your wife shall bear you a son, and you shall call his name Isaac; I will establish My covenant with him for an everlasting covenant, and with his descendants after him.’ ” God basically said to Abraham, “Yes, you got yourself a son, but no, I cannot bless a product of the flesh. Live by faith and let Me do it My way.”
Until Abraham didn’t know how to get a child, he didn’t need faith. He walked by sight first, obtaining Ishmael, but he was not the son God had promised. Only when it was completely impossible—when both he and Sarah were too old and when their schemes to get a child their own way had failed—did Abraham have the faith to let God do it His way.