In Genesis 15, Abram is finally made new. He no longer just a moral man, but a man who's heart has been transformed and redeemed through the act of belief. In verses 1 to 5 God comes to Abram in a vision, and Abram cries out with his inner doubts. He wonders how God's promises can come about when he still has no child and heir. Again, God reaffirms that Abram's own biological child will be the father of a great nation that will number more than the stars of the heaven. This time, Abram just believes. God knows His heart and the saving work is done.
And what is Abram's first act as a transformed man? To doubt. "Whereby shall I know?!" We humans don't change much over time. More than two thousand years later, Paul pens Romans 10:9 and 10, "That if though shalt confess with thy mouth the Lord Jesus, and believe in thine heart that God has raised Him from the dead, though shalt be saved." Mere verses later, he offers this reaffirmation in Romans 10:13. "He that calls upon the name of the Lord, shall be saved." It can be no coincidence that God's inspired Word first leads in the way of salvation and then reiterates the surety of it.
Nevertheless, God does not upbraid Abram for his question, but rather gives him instructions for worship. After he has obeyed, God reaffirms His promise and gives Him a level of insight into his future. He gives details and boundaries that those of us reading are blessed to see fulfilled in time.