The work of the Living God is very specific, very detailed, and very powerful. Though God certainly doesn't show all of His cards concerning His plans and the methods by which He will execute them, the scriptures do show that God reveals a good amount of insight in regards to His "big picture" plans. The best part about this truth is that God is able to perform and do as He desires with, or without the participation or faith of His people!
In Genesis 17:15-27 the Bible documents the last part of the dialogue between Abraham and God when God gave the command of circumcision. The scriptures show that God's promises were unconditional, but in order for Abraham and his offspring to receive the benefits associated with the fulfillment of God's promises, God required "circumcision." It is true that Genesis chapter 17 speaks of literal physical circumcision. However, New Testament scriptures confirm that the cutting away of flesh on the 8th day of a male child's life was to be symbolic of the commitment to follow God rather than the sinful desires of one's flesh. The covenant of circumcision was representative of sanctification.
Thus, as God gave the command of circumcision, the scriptures show God's efforts to bring Abraham closer to Himself in order that He would be able to fulfill His promises towards Abraham. God changed Abram's name to Abraham in order that Abraham would be identified as a child of the Living God and not of his earthly father. Genesis 17:15-27 also reveals that God sought to change Sari's name as well. God changed her name to Sarah, which means "princess" in Hebrew. God changed the names of both Abraham and Sarah in order to identify them both as His as a way to further confirm His willingness and faithfulness to fulfill His promises. As God sought to identify Abraham and Sarah as His own children, Abraham and Sarah should have felt closely connected to God and assured that as an earthly father would seek to bless his children, their heavenly Father would seek to do the same, and be able to in far greater ways.
Genesis 17:15-27 then shows that God reconfirmed His promise to bless Abraham with a child. Though Abraham had a 13 year old boy named Ishmael that was born by Sarah's maidservant Hagar, God provided details to His plan. When God said that Abraham would have a son, Abraham found it hard to believe. Abraham even asked God, "How is a 100-year old man and a 90-year old woman going to have a child?" Nevertheless, God assured Abraham that he would have a son the very next year. Abraham, still in disbelief, asked God if Ishmael could be the heir of God's promises to Abraham. However, God was clear and candid to say that Ishmael would not be the heir of Abrahamic Covenant. God was explicit to say that He would confirm His covenant with Abraham's younger son that would be born the next year - Isaac.
Today there is great debate in the Middle East about who owns what land. God promised Abraham the land of Israel in Genesis chapter 12, and confirmed the boarders and sureness of the promise in Genesis chapter 15 and 17. Yet the Word of God is also detailed to explain that, though Ishmael was the older son of Abraham, he was not the heir of God's promises to Abraham. God worked miraculous birth circumstances to bring Isaac into the world because God proclaimed that he would be the heir of the Abrahamic Covenant. The great nation, the Promised Land, and the blessing would go through the lineage of Isaac, not Ishmael.
However, God is not unfair. When Abraham showed a certain amount of grief over Ishmael not being the heir, God acknowledged that Ishmael would also be blessed. God promised that Ishmael would also be a great nation and a father of 12 tribes. Ishmael's greatness simply would be different than the greatness of Isaac. God also later promised that Ishmael would also receive a land inheritance. His land inheritance would simply be different than Isaac's. The point is, though the circumstances may have been confusing for some in regards to God's promises to Abraham, it is not the fault of scripture that people are confused. Genesis 17:15-27 plainly states that the promise of a great nation, the land inheritance and the blessing were to be confirmed through Isaac and his bloodline, not Ishmael.
Genesis 17:15-27 states that this promise that God confirmed through the unborn Isaac was STILL an everlasting covenant. For this reason, the Everlasting God would exercise His everlasting power to work a miracle so that a 100-year old man and 90-year old woman would have a child, one year after the events of Genesis chapter 17, just as God had promised. The scriptures show that no matter the circumstances, and no matter the faithlessness of God's people, God will do what He says in His time. It may have taken 25 years for God to fulfill His promise, but when Isaac was born as history confirms, the circumstances of his birth have God's fingerprints all over them. Therefore, if God's fingerprints are on the birth of Isaac, then God's promises are proven to be true, and God's power and faithfulness is confirmed, regardless of human understanding. The details of scripture reveal all of these truths with the intent to educate readers on the character and nature of the Living God so that God's children will learn to trust Him because ALL of His promises are golden and will come to pass!