Genesis 21:1 says, And the LORD visited Sarah as he had said, and the LORD did unto Sarah as he had spoken.  God had not forgotten His promise to Sarah, and He will never forget His promises to us.  The main promise of God is an everlasting relationship with Him to those who accept Christ as their Savior and Lord.  God has not forgotten this promise, no matter how long it may be before it is fully implemented.  Verse two states, For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him.   Sarah conceived, when the time was right for God.  I believe that one reason that God waited as long as He did was so that there could be little doubt that He was the One Who brought about Sarah's conception.  I believe, no matter the events leading up to a pregnancy, that God alone can give life.  I know many people would disagree, but this is my belief.  Verse three adds, And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac.  At least Abraham was obedient in naming the baby Isaac.  Just because we may have doubted God or have been disobedient to Him in the past does not excuse us from being obedient in the future.  Verse four says, And Abraham circumcised his son Isaac being eight days old, as God had commanded him.  Abraham also obeyed God's command to circumcise Isaac.  Even if we have carried out God's law in the past, as Abraham had with the circumcision of all the males in his household, he still needed to continue to obey God's command.  Even if we have obeyed God in the past, that does not excuse us from obeying Him today.  Verse five states, And Abraham was an hundred years old, when his son Isaac was born unto him.  This was no longer the norm as it was with the earlier generations, but this was a miracle of God.  Abraham and Sarah had both believed it was impossible, but with God nothing is impossible.  Verse nine declares, And Sarah said, God hath made me to laugh, so that all that hear will laugh with me.  If Sarah was talking about when she laughed when God told her she was going to have a son, then it was not God but Sarah's lack of faith that caused her to laugh.  If she was referring to her joy at having a son, then God did indeed bring her this joy.  Following God's plan for our lives should always bring us joy.  Verse seven asks, And she said, Who would have said unto Abraham, that Sarah should have given children suck? for I have born him a son in his old age.  Sarah was still looking at things through the eyes of the world and not through faith in God.  We should never be surprised when God fulfills His promises.  Verse eight states, And the child grew, and was weaned: and Abraham made a great feast the same day that Isaac was weaned.  When Isaac was weaned, Abraham made a great feast.  It does not say that this was a feast to celebrate God's goodness, but was more simply to celebrate Isaac.  We must always make sure that we give God the credit when good things happen in our lives.  Verse nine declares, And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking.  Hagars son was mocking the celebration for Isaac.  He was after all Abraham's first born, but I am sure had never had a feast of celebration for him.  Many people today, who do not believe in God and accept His gift of salvation will mock those who have accepted Christ..  Verse ten adds, Wherefore she said unto Abraham, Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac.  Sarah saw Ishmael, the son that she had made the plan for his birth, mocking the celebration of Isaac, she demanded that his mother and he be cast out.  Sometimes, when our plans, made without God's guidance come true, we may create problems for others and ourselves.  Hagar and Ishmael were really not guilty of anything, other than being a part of Sarah's plan to fulfill God's promise her way.  We must be patient and wait on God to fulfill His plan His way and never believe that we have a better plan.