Then the word of the Lord came to him: “This man will not be your heir, but a son who is your own flesh and blood will be your heir.”
Genesis 15:4 (NIV)
When God makes a promise to you and you don't see it fulfilled, that doesn't mean He won't keep it. God made a promise to Abraham to give him an heir who would come from his womb. Abraham believed in God, but that did not prevent him from failing in his faith when, to please his wife and try to fulfill God's promise himself, he decided to take Hagar as his wife in order to have the son promised.
Despite Abraham's mistake, God did not break His promise to give him a son as his heir. Some people, when they see nothing, their first reaction is to seek to satisfy this promise by their own means, unaware that by doing so, it is God and his grace that they are rejecting. The worst part is that they are not afraid to testify to what God has done for them. This is taking the name of God vain.
Abraham's example should teach us that all men are weak and can fall at some point in their walk with God. What we need to understand is that God used his story to teach us to trust Him when He makes a promise to us. Abraham didn't have the Bible as an example, he just had his faith. But you and I have his story to help us have faith and trust in God, even if His promises are slow to come true.
May the grace and the peace of God be with you all.