Nothing is impossible to the Lord

Genesis 17:15-26
God said, "Yes, but your wife Sarah will bear you a son, and you will call him Isaac. I will establish my covenant with him as an everlasting covenant for his descendants after him."
Twenty-four ears after the departure from Harran the birth of Sarah's son Issac is announced.
Circumcision was no new rite. In the nations around it marked admission to adult status in the tribe. Distinctively for Israel it was the outward sign - fro birth 0 of a special relationship with God; not simply a mark of ownership, but a token of the reality of all God's promises expressed in the repeated 'I shalls' of these verses. Circumcision also signified obedience to God on the part of his people.
Why?
God made impossible matter to possible, even Abraham was 99 years old. God has his time and his schedule. His word never fail.
Issac will success the covenant between God and Abraham, not Ishmael. God's promise is upon Issac.
How?
Nothing is impossible to the Lord, trust in Him. My God is the mighty God who created these world and gave us life and peace.