“And God said, ‘Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish My covenant with him for an everlasting covenant, and with his seed after him.” Genesis 17:19 (KJV)
Yes! Abraham had other children. His first was Ishmael; due to the impatience of Sarah and himself, thinking to take things concerning the promise of God into their own hands. God had better things in mind. Sarah was to have a son of promise. It was and is through God’s promise of a Saviour that the covenant was made with Isaac as the son of promise.
Just at the time you think it cannot happen; with God nothing shall be impossible. Jesus is the Son of the covenant of promise and grace. Paul the apostle has written by inspiration of the Holy Spirit…
“Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.” Romans 9:6-7
It is by the promise of Jesus, and no other way that one will be forgiven all sin, cleansed, and imputed all righteousness and have fellowship with God.