The verse contrasts a time 'before faith came' with a faith 'which should afterwards be revealed.' What's the difference between the faith of the past and the faith of the New Testament?
Paul isn't saying that people before Christ didn't have faith. Abraham, for example, is a prime example of faith (Romans 4). The distinction lies in the revelation of faith.
Clarity Over Shadows
Before Christ's coming, faith was understood more dimly, through types, shadows, and less explicit promises. The Old Testament economy, with its ceremonies and laws, pointed toward a reality that was not yet fully unveiled.
Christ: The Object of Revelation
'Faith' in this context often refers to the full unveiling of God's redemptive plan centered in Jesus Christ. When Christ came, the object of our faith was clearly revealed. What was once foreshadowed became substance. This 'revealed faith' is the New Testament era of salvation, where God's grace is openly displayed through Jesus.