As we come to Genesis 21 we shall find here a birth that we have been eagerly waiting for - Isaac the promised son of the covenant.
His name means “laughter”. And so it is. The birth of this boy brings new joy and laughter into the lives of this old couple. And it is by all accounts a miraculous birth. All this reminds us very much of the language of the time in which we find ourselves right now - the Christmas season-the remembrance of the birth of Christ our Lord. The story of Isaac in a sense prefigures the birth of our Lord Jesus Christ, who was long awaited, and born under mysterious circumstances, and who has brought laughter into the lives of many that have trusted in Him for their salvation.
Indeed, the prefigurement of Christ is found in every book of the OT. 
And yet, while it is a happy story, we shall also find that there are dark shadows lurking in the background. This portion of Scripture that begins with laughter (21:1-7) ends with a painful separation (21:8-21). We will learn lessons from both.
1. 21:1-7 : God is faithful
"The Lord did to Sarah as He had promised." Allow me to begin with a brief comment on the length of time it took for God to fulfill his promise to Abraham and Sarah. The promise of an offspring was first made in Chapter 12 when God called Abraham out of Ur of the Chaldees. At that time he was 75 (12:4) , and Sarah was 65 years old. It took 25 years to fulfill that promise (see 21:5)!
How are we to understand this? We remember that Abraham (like Adam) was foolish to listen to his wife when she suggested that he take her servant Hagar and have a son by proxy. Ishmael (who shall occupy our thoughts in the second half) was born as a result. He could of course not be the son of the promise. He was born as a result of the scheming of Abraham and Sarah. So, apart from this setback, what reason could we provide for this long waiting?
The answer is simply, “we don’t know!”
All we do know is that God chose to work in a mysterious way His wonders to perform, as He so often does! The mystery of Divine Sovereignty in the outworking of things is so well expressed in Isaiah 55:9 :
"For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts.”
Whatever we may think, the birth of Isaac was in the end a sovereign act of God, and, from God’s perspective it happened just at the right time. In that sense his birth was like that of Jesus, who is the ultimate seed or offspring of Abraham, and who in God’s providence was born at just the right time (Gal. 4:4 ESV In the fullness of time). In both cases the sons of promise were conceived despite human reason and ability, and through the supernatural activity of God.
There may be a number of people here today who have been waiting on God to fulfill what they believe is a divine promise from the Word of God to them. What shall we say to this?
- Be absolutely clear that you do have a clear promise from the Bible, and not from your own fancies.
- Remember also that a Bibles text taken out of context is a pretext.
God nowhere promises you in the Bible that you will have children if you had none so far.
God nowhere promises you in the Bible that you will recover from every illness and that you will be happy, wealthy and healthy all the days of your life.
God does not even promise that all the children of every believer will be converted.
But God does promise that He will “supply every need of yours according to his riches in glory in Christ Jesus.“ (Phil. 4:19)
He promises that, for any true believer, all things will work together for the good. (Rom. 8:28).
He has promised that nothing will separate us from His love in Jesus Christ (Rom. 8:31-39).
He has promised us that He will give the Holy Spirit to those who ask Him (Lk. 11:13).
He has promised us that if He has begun a good work in us he will complete it (Phil. 1:6).
He has promised to come again, and to take us to be where He is (Jn. 14:1-3).
So we can wait, standing on these promises of God, but the precise times of the fulfillment of His promises are not made known to us. By faith we believe and by faith we cling to the promises of God. God, our Father knows the best timing!
In this case God had given a specific promise to Abraham and Sarah and He was going to fulfil it. God is faithful. And now Sarah is happy… very happy (21: 6,7). The Lord has turned her reproach into a blessing, and Abraham makes it known in a very vivid way. He names their son , ‘Isaac’ … He laughs! (21:3)
The application of this truth is exciting. Whatever God promises, He will do. He will keep His word. And so we must learn to trust Him. All of us have our own challenges in trusting the Lord, but Genesis 21 has been recorded that you may know that God can be trusted. If we do not believe that God is faithful, then it will be very difficult for us to pray meaningfully.
C.H. Spurgeon often encouraged his congregation to study the promises of God and to pray according to the promises of God. Rehearse the promises of God. Meditate on them. Remember them, and trust God for the outcome. And do not shrink back (Hebr. 10:35-39). At long last we will see what we have hoped for, just as is the case here in 21: 1-7.
2. God's common Grace is kind to all mankind - but His love for His chosen people is without equal. (21:8-21)
In this portion of Scripture we find the doctrine of election applied. It begins with an old conflict in Abraham’s family life. As Isaac, the son of promise is weaned, Abraham makes a great feast for him, and we find that Hagar the mother of Ismael laughs – but not in a happy way , but in derision (21:9). At this point Sarah insists that Hagar leaves. Conflict is looming between the son of promise and the son born according to a sinful decision.
Paul makes a great deal of this story in his letter to the Galatians 4:21-31 where he tells us that the two boys, Isaac and Ishmael and their two mothers, Sarah and Hagar are allegories of two covenants. Sarah bore the son of promise, corresponding to freedom, living by grace. Hagar bore the son born according to the flesh, bearing children for slavery, living by law. The plain fact concerning them is that they cannot co-exist!
To put that into gospel language: The son of promise, Isaac corresponds to those living under the gospel. A gospel man or woman relies on that which Jesus Christ has done for them. If you have trusted in the Lord Jesus Christ, and have been born again by a second miraculous birth, then you belong to the line of Isaac, the son of promise. By contrast, Ismael, the son of the flesh, relies on himself. He judges himself by his own legal standards of self- righteousness. These two systems cannot co-exist, and so we find that Hagar and Ishmael must leave. This is not ultimately just a family squabble. It is about a profound biblical truth, and in Psalm 83: 4-6 we find an outworking of all this. Here the Psalmist shows us that the nations which are related to Israel are insanely jealous of them and therefore they conspire against Israel.
The distinction between Isaac and Ishmael is much more profound than we are able to understand at face value. It is a theological issue, and it is revealed, as we have already seen in Galatians 4:21 ff.
There are ultimately only two kingdoms in this world – and to these two kingdoms belong either the children born of the flesh or the children born of the promise. And in his letter to the Galatians Paul was saying, don't be surprised if the synagogue (the sons of the flesh, the sons of the Sinaitic law and hence ultimately the sons of Ishmael) is persecuting you.
And ultimately we shall see that Ishmael persecuted Isaac.
So do not be surprised if the unbelieving seed of Abraham is persecuting true Christians. That is Paul's argument and therefore he continues:
"But what does the Scripture say? Cast out the slave woman and her son, for the son of the slave woman shall not inherit with the son of the free woman. So brothers, we are not children of the slave , but of the free woman." (Gal. 4:30,31)
SENT OUT…. BUT NOT WITHOUT MERCY !
Sarah has asked Abraham to send Hagar and Ishmael away, and clearly this causes Abraham to be distressed. Abraham loved this boy deeply. In fact, when God came to Abraham to promise him the birth of Isaac, do you remember Abraham's response? Oh, that Ishmael might live before you. (17:18).
God, in response is not unkind. He promises Abraham that He will care for the boy and his mother, and to him are given promises (21:13,18). Here God says to Abraham, “Because I love you, Abraham, I will make him a great nation. I will protect him.”
All this is of great significance and later in Romans 9, Paul will use this story again to help us to understand that not all descendants of Abraham are sons of the promise :
… and not all are the children of Abraham because they are his (physical ) offspring, but through Isaac shall your offspring be named!” (cf. Rom. 9:6ff)
God is much more merciful to Ishmael than Sarah. And so often we think we are more merciful than God. But the truth is that God is much kinder. Hagar and Ishmael are not left destitute by any means . And this is true of all the unregenerate people of the world today. God is kind and gracious to those that deny him and curse him and ignore him. This will obviously have an end, when Jesus returns as the Judge making a final separation between the two kinds of people.
But this text makes it clear that there are only two kinds of people in this world - and so it is There are those that love and follow the true God who has made the heavens and the earth, and there are those that follow their own thoughts and their man made gods. The two are not compatible. So, God had to separate them, just as he had separated Abraham and Lot, so that his heart would be uncompromised and wholly God's. If God was going to establish His seed, it was going to be through Isaac alone.
Yes, it breaks Abraham’s heart, but God must break His heart to build his faith. There are so many lessons in this, but for the sake of the table set before us, I say only this:
When you come to the Lord Jesus (the ultimate Son of Promise) and you receive His righteousness , in the place of your own old insufficient self - righteousness, you become a son of Isaac, the son the promise. And you will leave your old ways of sin behind you. You will make a decisive break with ungodliness and continue with Jesus. Amen !
 In Genesis, Jesus is the seed of the Woman. In Exodus, He is the Passover Lamb. In Leviticus, He is the Priest, the Altar and the Lamb of Sacrifice. In Numbers, He is the pillar of cloud by day, and the pillar of fire by night. In Deuteronomy, Jesus is the Prophet like Moses. In Joshua, Jesus is the captain of our salvation. In Judges, He is our Judge and Law-Giver. In Ruth, He is our Kinsman and Redeemer. In 1 & 2 Samuel, He is our trusted prophet. In Kings & Chronicles, He is our reigning King. In Ezra, He is the rebuilder of the broken-down walls of our human lives. In Nehemiah, Jesus is our Restorer. In Esther, He is our Advocate. In Job, Jesus is our Ever-Living Redeemer. In the Psalms, He is our Shepherd. In Proverbs, He is our Wisdom. In Ecclesiastes, He is our hope of the resurrection. In the Song of Songs, He is our loving Bridegroom. In Isaiah, Jesus is the suffering Servant. In Jeremiah , He is the righteous who is wronged. In Lamentations, He is our weeping prophet. In Ezekiel, He is the one with the right to rule. In Daniel, Jesus is the fourth man in the fiery furnace. In Hosea, Jesus is the faithful husband.In Joel, He is the one who baptises with the Holy Spirit and with fire. In Amos, He is the restorer of Justice. In Obadiah, He is mighty to save. In Jonah, He is the Word of God to the nations. In Micah, He is the feet of one who brings good news. In Nahum, Jesus is our stronghold in the day of trouble.
In Habakkuk, He is God our Saviour. In Zephaniah, He is the King of Israel. In Haggai, He is the signet ring.
In Zechariah, He is our humble King riding on a colt. In Malachi, Jesus is the son of righteousness.
 God works in a mysterious way, his wonders to perform-a hymn by William Cowper (1731-1800)
 Note : needs … not wants . David was able to testify in Psalm 37:25 : “ I have been young and now I am old, yet I have not seen the righteous forsaken, or his children begging for bread.”