This means that He cannot fellowship with sin. He must demand the same perfection of His creatures. However, the righteousness of God is freely available to all who believe. God is Just, and cannot be unfair. His justice demands that disobedience against His laws be punished.
Justice administers the penalty that right-eousness demands.
For we know him that hath said, Vengeance belongs unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people. It is a fearful thing to fall into the hands of the living God. God's justice is satisfied because of His grace provision of redemption. The Lord Jesus Christ, through his substitutionary, effective death on the Cross, transferred the guilt of the sinner onto Himself, thus satisfying the justice of God.