ASV notes on Genesis 17:15-16

When you have found God’s will for your life, God talks to you, and tells you what to do, if you haven’t the will of God in your heart, you are told things from other people, how do you know if they are telling you the words of God directed to you. God spoke to Abraham, he had first-hand knowledge, and there were nobody between him and God. That’s how it is to walk with God every day, God speaks to you in His silent way and you feel safe, secure and good. 

At first Abraham doubted that Sarah could have a child and start his descendants and meant that Ishmael could be the one. Abraham talked to God about it and God assured him that his descendants would come through Sarah. God is great and almighty but we can talk to him, we can have a personal conversation with Him.