"Abram believed the Lord, and he credited it to him as righteousness." NIV translation
Abram accepted God's answer and believed that God would keep His promise. This is a picture of salvation through Jesus Christ as, like Abram, we must just accept God's answer to our problem of sin and believe that Jesus paid for it. You can imagine that Abram could not understand how he was going to have so many children especially since he was getting old. In the same way, we don't have to understand how the Son of God could come down and pay our sin debt. We just have to believe! We do not sin by questioning God; we sin by not believing His answer.
"He also said to him, 'I am the Lord, who brought you out of Ur of the Chaldeans to give you this land to take possession of it.' But Abram said, 'O Sovereign Lord, how can I know that I will gain possession of it?'" NIV translation
Now, we see that Abram has another question. God reminds Abram that he was brought from Ur to Canaan to take possession of the land. Abram did not see it happening even though he had just returned from winning a great battle and retrieving his nephew. He asks God for proof and we see this happen throughout the Bible. Is it wrong to ask for such proof?
"So the Lord said to him, 'Bring me a heifer, a goat and a ram, each three years old, along with a dove and a young pigeon.' Abram brought all of these to him, cut them in two and arranged the halves opposite each other; the birds, however, he did not cut in half. Then birds of prey came down on the carcasses, but Abram drove them away." NIV translation
This may seem like a strange answer to us but God's instructions are basically for Abram to prepare for the contract signing. The way that agreements were made between two parties began with offering a sacrifice and dividing it in half. We also see that Satan (using the birds of prey) wanted to stop the covenant but Abram drove the birds off.
We may ask what this means to us today and the basic answer is that God is still the God of contracts. Jesus was the sacrifice provided at our salvation contract signing and we must hold on to that fact. Like the birds of prey with Abram, Satan tries to send false teachings to rob us of the assurance that we are saved.