The Birth of Isaac
Finally, after 25 years, God fulfilled His promise to provide a son and heir to Abraham and Sarah! The wait had been difficult, sometimes dispiriting, sometimes frustrating. But, true to His word, God did just as He promised—and just when He promised the previous year (Genesis 18:10 Genesis 18:10And he said, I will certainly return to you according to the time of life; and, see, Sarah your wife shall have a son. And Sarah heard it in the tent door, which was behind him.
American King James Version×, Genesis 18:14 Genesis 18:14Is any thing too hard for the LORD? At the time appointed I will return to you, according to the time of life, and Sarah shall have a son.
American King James Version×).
But the birth of the promised son did not lead to peace and joy. Instead, the fruit of Abraham and Sarah’s attempt to fulfill God’s promise through Hagar was now beginning to be borne. Strife rent the household of Abraham, with Sarah seeking to ensure Isaac’s preeminence and resenting Abraham’s love for Ishmael, his other son. Although the narrative presents the entire transaction in a relatively brief space, it is likely that the tensions in the household had been building for quite some time. Ishmael’s ill treatment of Isaac was merely the straw that broke the camel’s back.
Abraham was distressed by the entire affair. He truly loved Ishmael (Genesis 17:18 Genesis 17:18And Abraham said to God, O that Ishmael might live before you!
American King James Version×) and, given God’s pointed mention of Hagar in Genesis 21:12 Genesis 21:12And God said to Abraham, Let it not be grievous in your sight because of the lad, and because of your female slave; in all that Sarah has said to you, listen to her voice; for in Isaac shall your seed be called.
American King James Version×, Abraham may have had tender feelings toward her. He probably tried everything he could to keep peace in the house. But it was to no avail. In this circumstance, God told Abraham to heed the words of Sarah. Whether or not Sarah had a right to feel and behave as she did, God’s plan necessitated the separation of Ishmael from the household.
In requiring the separation, however, God reassured Abraham that Ishmael would be blessed, “because he is your seed” (verse 13). In other words, although God had not obligated Himself to provide for Ishmael, nor bless him, God would graciously bless Ishmael because God loved Abraham and Abraham loved Ishmael. God’s grace sometimes falls on others because of His love toward His people. When we become His children, God’s love and affection is extended to more than simply ourselves. Because He loves us and we love others, God, for our sake, sometimes extends His protection and blessing to those we love. This is born out explicitly in 1 Corinthians 7:14 1 Corinthians 7:14For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy.
American King James Version×, where Paul tells us that an unbelieving spouse is sanctified by the believer—an extension of God’s love toward us. So, though we are separated from the world by the plan and call of God, nevertheless we are given the sure knowledge that because of our separation to God, our unconverted loved ones will often share in the overflow of God’s grace.
We should also stress in this context the kinds of problems that can arise whenever we depart from God’s pattern for marriage—that a man and woman unite for life in a loving monogamous relationship (Matthew 19:5-6 Matthew 19:5-6  And said, For this cause shall a man leave father and mother, and shall join to his wife: and they two shall be one flesh?
 Why they are no more two, but one flesh. What therefore God has joined together, let not man put asunder.
American King James Version×). As we see from the example of Abraham, Sarah and Hagar—here in Genesis 21 and earlier in chapter 16—relationships contrary to this pattern lead to heartache, jealousy, bitterness and misery. We see many of the same problems again when we come to the life of Abraham’s grandson Jacob. These stark examples should remind us of the kinds of consequences we saddle ourselves and others with when we decide to ignore God’s laws and instructions.