Justification is a legal standing with God based upon Christ's death and resurrection and our faith in Him. The word Paul uses (dikaioo), comes from Roman legal courts meaning to declare to be righteous, or to pronounce righteous. Therefore, justification is the legal and formal acquittal from guilt by God who is Judge. It is the pronouncement of the sinner as righteous, who believes on the Lord Jesus Christ.
Let's suppose for a moment that I died tonight and stood before the Lord God who is the Supreme Judge of the Universe. No doubt He would ask me, "Wil Pounds, why should I let you into my heaven? You are a guilty sinner. How do you plead?"
My response would be, "I plead guilty, Your Honor."
My advocate, Jesus Christ, who is standing there beside me speaks up for me. He says, "Your Honor. It is true that Wil Pounds is a grievous sinner. He is guilty. However, Father, I died for him on the cross and rose from the dead. Wil Pounds has put his faith and trust in Me and all that I have done for Him on the Cross. He is a believer. I died for him, and he has accepted Me as his substitute."
The Lord God turns to me and says, "Is that true?"
I will respond to Him, "Yes sir! That is the truth. I am claiming the shed blood of Jesus Christ to cleanse me of all my sins. I have put my faith in Jesus to save me for all eternity. This is what You have promised in Your Word. Jesus said, 'For God so loved the world (and this includes Wil Pounds), that He gave His only begotten Son, that whosoever believeth in Him should not perish, but have everlasting life.'"
The Lord God responds: "Acquitted! By order of this court I demand that you be set free. The price has been paid by My Son."
Furthermore, I get to go home and live with the Judge!
Justification means that at the moment of salvation God sovereignly declares the believing sinner righteous in His sight. The believing sinner is declared to be righteous in His standing before God. From that moment on throughout life, through death, that sinner who has believed is now and forever right before God. God accepts him, and he stands acquitted of his sins.
A man is not justified by the works of the Law but through faith in Christ Jesus, even we have believed in Christ Jesus, that we may be justified by faith in Christ, and not by the works of the Law; since by the works of the Law shall no flesh be justified (Galatians 2:16).