As one reads the life of Abram whose later name became Abraham, God promised to Abraham that his wife will have a son.
In Genesis 18, God said this to Abraham, “Then one of them said, “I will surely return to you about this time next year, and Sarah your wife will have a son.” Now Sarah was listening at the entrance to the tent, which was behind him. 11 Abraham and Sarah were already very old, and Sarah was past the age of childbearing. 12 So Sarah laughed to herself as she thought, “After I am worn out and my lord is old, will I now have this pleasure?” 13 Then the Lord said to Abraham, “Why did Sarah laugh and say, ‘Will I really have a child, now that I am old?’ 14 Is anything too hard for the Lord? I will return to you at the appointed time next year, and Sarah will have a son” (NIV).
In Genesis 21, God fulfills his promise to Abraham and Sarah, “Now the Lord was gracious to Sarah as he had said, and the Lord did for Sarah what he had promised. 2 Sarah became pregnant and bore a son to Abraham in his old age, at the very time God had promised him. 3 Abraham gave the name Isaac[a] to the son Sarah bore him. 4 When his son Isaac was eight days old, Abraham circumcised him, as God commanded him. 5 Abraham was a hundred years old when his son Isaac was born to him“ (NIV).
Based on those two passages, here are thoughts on how Abraham believed that God would provide a lamb for the sacrifice:
A. God commanded Abraham to offer a sacrifice to him at the mountain he will show him. The object of that sacrifice was to be his son.
B. Abraham recalled God’s promise that it will be through his seed that the earth will be blessed. Isaac was God’s gift to him.
C. Abraham may have further reason three possible scenario:
1. Abraham sacrifices Isaac as the offering and he dies. God could give him another son through Sarah.
2. Abraham sacrifices Isaac as the offering and God resurrects him to life.
3. Abraham sacrifice is intervened by God while God providing a lamb on the altar.
In the three possibilities, Abraham believed that nothing is impossible with God. Did God really intended Abraham to kill his son? Abraham may have believed that God will intervene or God will give him another son. Thus Abraham and Isaac came to the place. Abraham didn’t bring a lamb or goat. He took a step of faith trusting that God will do what is best for him. As he journey to that place, he expressed faith that God will provide a lamb for him. In all of Abraham’s experience, God has never asked him to do something that was immoral. God was willing to spare Sodom if 10 righteous people were found. Would God allow him to kill the promised child from him? Abraham rested in the goodness and mercy of God. He said to his servants that he and his son will return. He didn’t believe that he will return alone. Since God commanded a sacrifice, then he reasoned that if it is not his son, then God will proved a substitute.
SUMMARY: Abraham relied on the character of God and the promise of God that Isaac is his heir.