Abraham was an old man with no heir. He did not know how an old man with an old wife could bear a child. Sarah too had grown doubtful. Both laughed at the promise of God, but God would show them His power to overcome what seems impossible to people. Abraham believed God’s promise to have heirs, but he did not yet know how God would keep His promise.
Abraham first considered his servant Eliezer of Damascus to be his heir, but God said no (Genesis 15:2-4). Later Abraham wished Ishamael might be the one to “live before Thee” (17:18). But God had different plans. There was another to be born of Sarah who would be his heir. Only the son of promise could fulfill God’s plan (Galatians 4:21-31).
Many times in life we don’t know how God will accomplish His purpose. We think we have a plan our own, like Sarah did with Hagar; but God’s ways are higher than ours (Isaiah 55:8-9). God doesn’t always reveal in the beginning how He will accomplish His promises. Our task is to believe He will even when we don’t understand. God doesn’t need our help to figure a way to accomplish His purposes, but He does need our trust and willingness to follow.