Summary: No doubt, it had been a difficult 25 years, but now the new parents were enjoying their new baby son.Remember though, that God had also promised Abraham that his descendents (through Isaac) would inherit the country they were living in to be their very ow
Purpose: To focus on the eternal nature of God.
Aim: I want the listener to marvel at God's greatness because He is eternal.
INTRODUCTION: Abraham received two amazing promises from God: he would have a son in his old age and his descendents would inherit a land that would belong to them forever. Genesis 48:4 and He said to me, 'Behold, I will make you fruitful and numerous, and I will make you a company of peoples, and will give this land to your descendants after you for an everlasting possession.' (NAU)
As we come to Genesis Chapter 21 Abraham had finally received the son that God has promised. Abraham and his wife Sarah had to wait almost 25 years after God promised a son before Isaac was born. Abraham and Sarah not only had to wait a long time, they had to wait for the impossible to happen--Sarah was 90 years old and Abraham was 100 years old when Isaac was born.
No doubt, it had been a difficult 25 years, but now the new parents were enjoying their new baby son. Genesis 21:5--8 Now Abraham was one hundred years old when his son Isaac was born to him. Sarah said, "God has made laughter for me; everyone who hears will laugh with me." And she said, "Who would have said to Abraham that Sarah would nurse children? Yet I have borne him a son in his old age." The child grew and was weaned, and Abraham made a great feast on the day that Isaac was weaned. (NAU)
Remember though, that God had also promised Abraham that his descendents (through Isaac) would inherit the country they were living in to be their very own, but Abraham and his family were still living in tents.
For the time being Abraham dug a well and pitched his tent in a place he named Beersheba. This would later be known as the most southern town in Israel. It appears from the next chapter that Abraham and his family lived in Beersheba for at least 15 years.
In verse 33, we find a new name for God: "The Everlasting God" or the Hebrew name: "El Olam = (Genesis 20:13); God of eternity; KJV the everlasting God. The God without a beginning; the God who never will cease to be; the God who will never grow old; the God to whom eternity is what present time is. Describes God as He who extends beyond our greatest vision of who we think God is (no matter how great our concept of God is, He is always greater)." 
The fact that God is eternal is a difficult idea to understand. How long is forever? We may joke and say nothing lasts as long as a new breakfast cereal you can't stand or nothing wears as long as that ugly carpet whose hideous color you hate. Someone else put it this way: Nothing on this earth lasts forever--with the possible exception of public broadcasting pledge weeks. These things may seem eternal, but we know that they aren't.
Only God is really eternal. Since God is eternal, He is not limited by the constraints of time. He has no beginning, no end. He is never in a hurry and He is never late. The past, present, and future are all the same to God. This also means that nothing could ever take God by surprise, and that God has never learned anything since He has eternally known everything. Romans 11:33 Oh, the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments and unfathomable His ways! (NAU)
"God's sovereignty extends through the passing of time and beyond our ability to see or understand." 
An illustration may help us. Picture a large table with dozens of various objects scattered all over it. Imagine how an ant would view the things on this table. It would crawl to one object and explore it and then move on to the next object. We could say that the ant would go from one thing or event to the next on that table. We, on the other hand, could look down on the table and see everything all at one time.
As we go through life, we experience it as a series of events, one after the other. God, though, sees everything all at once. This helps us to understand a little of what God meant when he told us that "My thoughts are not your thoughts, nor are your ways My ways," declares the LORD. "For as the heavens are higher than the earth, so are My ways higher than your ways and My thoughts than your thoughts." (Isaiah 55:8--9 NAU)
It was this knowledge of God that prepared Abraham for the test that he would go through in the next chapter where God will ask him to sacrifice his only son. An eternal God can do anything: Genesis 22:8 Abraham said, "God will provide for Himself the lamb for the burnt offering, my son." So the two of them walked on together. (NAU) Habakkuk 1:12 Are You not from everlasting, O LORD, my God, my Holy One? We will not die. .... (NAU)