Justification is a once for all time event where when we put faith in Jesus and his blood spilled for us and acknowledged Him as our Lord and Saviour we were declared righteous by God. Justification is a legal/judiciary term in the context used here in the bible and means that we are declared by God, "Not Guilty" and have Jesus' righteousness imputed to us.

Romans 5:1 Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ. 2 Through him we have also obtained access by faith into this grace in which we stand, and we rejoiced in hope of the glory of God.

Since this righteousness is a gift by God's grace and did not come to us because of what we did we cannot lose it because of what we do either. Righteous is what we ARE! Our righteousness is permanent and from that base, by God's grace, we are enabled to stand in the sight of God and do the works He has prepared for us.

That's something to rejoice about!