To borrow Jesus' illustration from the Gospel lesson, relying on our own righteousness is like building a house on sand. Paul's point is that the righteousness of God is not just His standard, but is also, through faith in Christ, His free gift to us. It is not something we can attain. God's righteousness is ours apart from the law altogether. It is His free gift to us along with the forgiveness of sins.
Forgiveness is a pardon in spite of the evidence.