In what would have been a powerful illustration to the Jewish Christians in the Romans church, Paul makes the point that just as was true in the life of Abraham, we are saved by faith not works.
Everyone has a definition of “faith.” This chapter has a pretty good one too:
He [Abraham] didn’t waver in unbelief when faced with God’s promise [of a son even though he was approaching 100 years old]. Instead, he grew strong in faith and gave glory to God, being fully convinced that God had the power to accomplish what he had promised [even though it defied logic]. (4:20-21)
Faith is believing that God can do something even though it is entirely against all odds.