Genesis 25:21 (NKJV)
Now Isaac pleaded with the Lord for his wife, because she was barren; and the Lord granted his plea, and Rebekah his wife conceived.
We learn some interesting things about prayer in this passage. Isaac was fervent in his prayer and it was an intercessory prayer for his wife. Now we know that God had already promised through Abraham, that it would be thorough Isaac that he would have descendants more numerous that the stars. So, why is it that Isaac needed to pray and ask God to fulfill what he had already promised? We can conclude from this example and from many others in the Bible that this is exactly how God accomplishes his sovereign will—through the prayers of his people. One benefit of this is that after much waiting on the Lord, neither we nor Isaac could conclude that these children were just the result of the laws of nature since it is natural for a woman to have babies. But because of Rebekah's barrenness for 20 years, the credit and glory must go to God as both answered prayer and as part of accomplishing his will and promise through grace. Additionally, Isaac and Rebekah had to exercise faith and patience and dependence on God.
Ephesians 1:11
In Him also we have obtained an inheritance, being predestined according to the purpose of Him who works all things according to the counsel of His will
God’s sovereign will is accomplished through answered prayer. But, there is more to be learned about prayer in the next verse in this story.
Genesis 25:22-23
But the children struggled together within her; and she said, “If all is well, why am I like this?” So she went to inquire of the Lord. And the Lord said to her: “Two nations are in your womb, Two peoples shall be separated from your body; One people shall be stronger than the other, And the older shall serve the younger.”
Notice where Rebekah goes to find out what is going on with her. We should always turn to the Lord first with all of our questions. But notice God's answer to Rebekah. God’s will about Rebekah's situation and about the future is revealed through answered prayer.
One of the great and difficult questions about the mystery of prayer and sovereignty is this: If God knows everything that is going to happen, and he is unchanging, how is it that prayer can change God? I think this little episode goes a long way towards answering that question. God answers our prayers to both accomplish and to reveal his sovereign will.