You have to understand what justification means. Again, it's not some get-out-of-hell-free-card as long as I recite some truths, perhaps passing a divine litmus test of professed beliefs. Justification involves the repentance you've cited above. It's a turning away from sin/the world and a turning to God and His values. If justification only means forgiveness of sin, then why do I even need to confess and be purifed of unrighteousness? Grace means God's life in us as we're now directly connected to the Vine. It's a life, a state of being, not just a legal status. If we turn away from that life, from that grace, from that "gift of righteousness", from God the Spirit indwelling us, then we've failed to remain in Him, and that metanoia, that change of heart/mind, must take place again with repentance forthcoming.