But Sarai (Abram's wife) was barren;

she had no child (Genesis 11 30). The genealogies in  chapters 9 though 11,  largely follow the godly line of Shem (Noah's son).   Shem's sons  are listed “according to their families, according to their languages and lands, according to their nations (Genesis 10:31). Note that  this implies exponential increase. Abram's  father Terah was at least ten generations from Noah (Luke 3:34-36), and Sarai was Abram's half sister, the daughter of his father, but not his mother (Genesis 20:12).  Idol worship had crept into the family (for example in Abram's nephew Laban Genesis 11:27; 29:5;  31:26, 30).

God was calling Abram out of this to continue the godly line. His  covenant with Abram included the promise that “in you all the families of the earth shall be blessed” (12:1,2). But of course Sarai was barren.  Rightly or wrongly, she understood that the Lord had prevented her from bearing children” (Gen 16:2; see also Gen 30:2). But how then was Abraham to become the father of faith,  the father of all who believe, the father of all to whom righteousness would be credited  (Genesis 15:16; Romans 4:11)?  Well, faith is not just about intellectually believing in God, faith is about overcoming  (1 John 5:5). But in order to be an overcomer, you have to have something to overcome. So yes, Sarai was barren, but all things are possible to the one who has faith (Mark 9:23).

I was asked just recently why we need to pray.  I mean God can simply do what He wants, He is Sovereign!  But God did not create man to be like pet mice in a cage, so that he could watch and be amused. He crated man in His own image to have fellowship with Him, and to co-labour with Him in bringing in the Kingdom (1 Corinthians 3:9).  Yes God is Sovereign,  and He could do anything He wants, but most of the time it's true that “without God, man cannot, without man,  God will not.” In other words He has chosen to largely restrict His intervention in creation to the prayers, and activity of His children.  This is especially true with respect to bringing in the kingdom.  

The other thing, is that we tend to be fixated on the end result, in Abraham's case, a son. But the Lord has purpose in the delay, and He has many things He wants to grow in us,  on the journey.  In particular faith. Faith is not believing what you know's not true, about being in denial of reality. Abraham “faced the fact that his body was as good as dead--since he was about a hundred years old--and that Sarah's womb was also dead.  Yet he did not waver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God” (Romans 4:19, 20). As we shall see, Abraham's patience and faith are far from perfect, but thank God that when we confess our mistakes the Lord chooses not to remember them (Hebrews 10:17).  In particular, when we don't give up on His promises, our faith is strengthened.

Father, these things are written for our instruction, so that through perseverance and the encouragement we might also grow in the hope  and faith of Abraham (Romans 15:4). Help us, help me, to learn to be content on the journey,  knowing You are at  work all things for our good as we continue to believe. Help us like Abraham to never, never,  never give up, in Jesus Name Amen