Our love for Christ and following Him matters. It not only affects us, but all those around us, and particularly our children. Isaac was the inheritor of the Lord's promise because of God's grace and goodness, but also his father Abraham's obedience played a part.
God will meet us where we are. When we doubt and struggle in faith, God is gracious to come and remind us (again) of His promises. And He is faithful. Promises He made to our ancestors that involve us will be fulfilled! God has rich, personal promises He longs to make and accomplish in our life as we look to Him. He is not limited by time or history. What He has said, He will accomplish.