Paul makes a great deal out of Abraham's faith. We are to be saved by a similar faith, and not by works.
However, I don't think Abraham's type of faith validates what Paul claims, nor can modern Christians have the same sort of faith Abraham had. If we take all stories in Genesis about Abraham at face value, then he never had to worry about "having faith".
Our first real introduction to Abraham comes in Genesis 12. He immediately has God speak directly to him, promising him blessings and curses. As Abraham travels, God again speaks directly to him, telling him he will give Abraham and his decendants all of the land he can see. He is told this several times. God speaks to Abraham multiple times, giving clear direction and guidance. Abraham even gets to argue with God regarding the fate of Sodom and Gomorrah.
Once Abraham seems to completely doubt God's promise. In Genesis 17:17, he laughs at God when got tells him that Sarah will have a child.
So we cannot possibly have the same kind of faith Abraham had. Why? Two reasons.
First, the method of communication. Abraham had a direct conduit to God, with specific instructions for what to do. We do not have this sort of revelation. We have a book that was written 2,000 years ago, and ratified by people 1600 years ago. Our faith is not directly to God, it is the intermediaries that have been used to communicate over the past thousands of years. Abraham had faith in his own sensory data and experiences. It is as different as one person having sustained, multiple visions of aliens and another reading about some group on the internet who claims to have seen aliens.
Second, the type of data we are expected to believe. We are asked to believe a certain historical event occurred 2,000 years. Abraham was asked to obey specific instructions, so that he would see certain benefits in his lifetime. He also had the benefit of witnessing a miracle (birth of Isaac). We do not have either sort of immediate reassurance, with our hope being in an impossible to verify life after death. This would be as different as a child believing they will get ice cream if they clean their room and another child believing they will have an invisible, never accessible bank account set up in their name if they acknowledge that the moon landing was a hoax.
These two together show that Abraham had a completely different faith experience than is even possible for us to experience, there for Paul is wrong to base salvation on Abraham's type of faith.