Charles Spurgeon Commentary Genesis 15:1-3

"After these things the word of Jehovah came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, [and] thy exceeding great reward. And Abram said, O Lord Jehovah, what wilt thou give me, seeing I go childless, and he that shall be possessor of my house is Eliezer of Damascus? And Abram said, Behold, to me thou hast given no seed: and, lo, one born in my house is mine heir." — Genesis 15:1-3 (ASV)

Perhaps he did not doubt the promise, but he wanted to have it explained to him. He may have wondered if it meant that one born in his house, though not his son, was to be his heir; and that, through him, the blessing would come. He takes the opportunity of making an inquiry, that he may know how to act.

At the same time, there does seem to be a clashing between Abraham's question, What wilt thou give me? and the declaration of God, I am thy shield, and thy exceeding great reward. There is a great descent from the language of the Lord to that of the most stable believer, and when you and I are even at our best, I have no doubt that, if all that we think and say could be recorded, some of our fellow-believers would feel that the best of men are but men at the best, and that God's language is after a nobler fashion than ours will ever be, until we have seen his face in glory.