This righteous relationship with God is not by particular law, works, or any tradition, but through living in Christ or following his faith. In this light, justification is not made once and for all. It must be sought and lived out until the end.
Pistis christou is also important in Rom 3:22, in which Paul summarizes his threefold gospel: God's righteousness, Christ's faith, and Christian participation in his faith.
See this blog for a difference between pistis christou and pistis en christo.
For more about this idea of faith, see this book below: