Genesis 17:15 says, And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be.  God now speaks to Abraham about Sarai, his wife.  God did not speak to Abraham about Hagar being Abraham's wife, because to God she wasn't.  A marriage is between one man, one woman, under God's guidance, for a lifetime.  Sarai would now be Sarah.  Verse sixteen states, And I will bless her, and give thee a son also of her: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her.  God promised that Sarah would have a son, and she would be the mother of nations.  God's promise to Abraham had always been for Sarah and Abraham, and no one else.  When we attempt to force God's plan into action because we are impatient we can only make a mess of things, as Abram and Sarai, as they were then, did with Hagar.  Verse seventeen, Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear?  Abraham fell on his face, but he laughed at God.  He thought it impossible for Sarah and himself to have a child at their ages.  We may sometimes look like we are worshipping God while at the same time feeling that what He is asking us to do is impossible, but we should never laugh at God's promises.  Verse eighteen says, And Abraham said unto God, O that Ishmael might live before thee!  Abraham still thought that Ishmael was the answer to God's promise.  We should never attempt to force our plans onto God.  Verse nineteen declares, 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.  God once again told Abraham that Sarah was going to have a son, and he would be named Isaac.  When God speaks to us, we should listen to what He tells us and not change what He tells us to what we believe possible.  Verse twenty adds, And as for Ishmael, I have heard thee: Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation.  God blessed Ishmael as well.  Ishmael was not responsible for the facts of his birth and was not to be punished by God.  Verse twenty-one concludes, But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year.  God said His promise to Abraham would be fulfilled through Isaac.  We should always make sure that our plan is God's plan, and when He reveals it to us, we must simply act in faith.  Verse twenty-two states, And he left off talking with him, and God went up from Abraham.  Like with Abraham, when God has told us what He wants to say to us, there should be no reason for Him to say more.  Verse twenty-three states, And Abraham took Ishmael his son, and all that were born in his house, and all that were bought with his money, every male among the men of Abraham’s house; and circumcised the flesh of their foreskin in the selfsame day, as God had said unto him.  Abraham carried out God's commandment about circumcision.  Verse twenty-four says, And Abraham was ninety years old and nine, when he was circumcised in the flesh of his foreskin. Verse twenty-five adds, And Ishmael his son was thirteen years old, when he was circumcised in the flesh of his foreskin.  Both Abraham and Ishmael, who was now thirteen years old, were circumcised.  Verse twenty-six states, In the selfsame day was Abraham circumcised, and Ishmael his son.  Verse twenty-seven adds, And all the men of his house, born in the house, and bought with money of the stranger, were circumcised with him.  Abraham carried out God's instructions in this case without attempting to change them.  We may fail to carry out God's command at some time, but if we do that does not excuse us from doing so later.