Romans 4:18

Who in hope believed against hope, to the end that he might become a father of many nations, according to that which had been spoken.

Paul in these words was showing the quality of Abraham's faith, which consisted in this, that he truly believed God, even though God's words were contrary to all natural and human expectations. Abraham was old, and Sarah's womb was dead, but he believed God, believing that, indeed, he would become the father of many nations. This quality of believing in "things not seen as yet" was made the theme of the entire eleventh chapter of Hebrews; from which it is to be concluded that genuine faith accepts what God has said, no matter what considerations of human wisdom and experience seem to nullify it. Such is Abrahamic faith.

So shall thy seed be ... is a quotation from Genesis 15:5, where is recorded God's commandment for Abraham to number the stars, saying, "So shall thy seed be." Brunner's definition of true faith is thus:

The existence of faith in God's promise is completely clear only where God's promise runs counter to human expectation and calculation. Whether one really relies on God's word alone becomes manifest only where God's word is not supported by any rational basis, but where, on the contrary, it is opposed to what one must consider probable on the basis of reason.[15]

Of course, Abraham truly had such faith.