Believers in the gospel of Christ, as laid out in 1 Cor. 15:1-4 KJV, have submitted themselves "unto the righteousness of God"(Romans 10:3 KJV).
We now have the righteousness of God, Romans 3:21-25 KJV, 4:5, secured through the death, burial, and resurrection of the Lord Jesus Christ-again Cor. 15:1-4 KJV.)
The LORD God has justified us forever only because He sees us "in Christ": Acts 13:43 KJV; Romans 3:24-28 KJV, Romans 4:25 KJV, Romans 5:1 KJV, Romans 5:9 KJV, Romans 8:30 KJV; 1 Cor. 6:11 KJV; Galatians 2:16 KJV, Galatians 3:24 KJV; Titus 3:7 KJV. This is an permanent, irreversible reckoning, decalared by the LORD God. Justification is that judicial act of a holy, just LORD God, by which, on account of the Lord Jesus Christ, to whom I am united("in Christ") by faith, and He is in me, united, He declares me to be no longer exposed to the penalty of the righteous requirements of the law, but restored to divine favor. I have been declared righteous. Justification is an act, a declaration, not a process. Upon believing 1 Cor. 15:1-4, I was given a free gift(Romans 5:18 KJV)-a righteous standing before a holy LORD God. This gift, by definition, has nothing to do with what I have done, or who I am. No amount of self-effort, or "good" works could ever bring one to justification. I did not justify myself-it is the LORD God who justifies. This gift is unchanging, cannot be given back. Once the LORD God declared me righteous, only because of what His only begotten Son did for me, what He merited, the sin/sins question regarding me has been settled once for all.
However, justification is more than forgiveness, since implicit in forgiveness, is guilt and cancellation/removal of sin, of the sin of "just doing what is required," while justification is "not guilty" at all, and the bestowing of the merit, and standing of the Lord Jesus Christ. Justification is not equivalent to being pardoned. A pardoned criminal is still a criminal. That is, a forgiven, pardoned thief, murderer,.......is still a thief, murderer....Justification removes the guilt. The LORD God thus not only "forgets" my sin, as though it never happened, but does not consider me a sinner anymore, because He sees me "in Christ".
Because I am "in Christ Jesus", I have been "...made the righteousness of God in him..."(1 Corinthians 1:30 KJV, 2 Corinthians 5:21 KJV). I am "... found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith..."(Phillippians 3:9 KJV)
All my sins(plural-1 Cor. 15:1-4) were forgiven at Calvary and the resurrection 3 days later:
"In whom we have redemption through his blood, even the forgiveness(emphasis mine-past tense)of sins" Col. 1:14 KJV
"And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven(emphasis mine-past tense) you all trespasses" Col. 2:13 KJV
"Forbearing one another, and forgiving one another, if any man have a quarrel against any: even as Christ forgave(emphasis mine-past tense)you, so also do ye." Col. 3:13 KJV
"In whom we have (present tense-emphasis mine) redemption through his blood, the forgiveness(emphasis mine-past tense) of sins, according to the riches of his grace" Eph. 1:7 KJV
"And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven(emphasis mine-past tense) you" Eph. 4:32.
Romans 5....."sin on a person."
In Romans 5:12 KJV Paul is no longer dealing with sins(plural), but rather with the source of sins, indwelling sin. Romans 5:12 KJV is speaking of identification, who we were in history, "positionally." We were all identified, reckoned, with the source of humanity-"in Adam". When Adam sinned, he died spiritually, and thereby died to God, and so did I- I died with him. When he became flesh, so did I-again, "in him." When he was judged/condemned, I was also judged in him.
Unlike the result, product of sin, sins(plural), the source, sin could not be forgiven, as it would be sin still. Again, the analogy-a forgiven thief is still a thief! Hence, sin had to be condemned/judged in death, Thus.....
"...God sending his own Son in the likeness of sinful flesh, and for SIN(emphasis mine-not sins), condemned SIN(emphasis mine) in the flesh."(Romans 8:3 KJV).
Thus, our sins, plural, were forgiven by the principle of substitution(the doctrine being woven through the book):
"...Christ died for our sins..."-1 Cor 15:3 KJV
"...gave himself for our sins...."- Gal. 1:4 KJV
But our sin, singular, was condemned/judged by the principle of identification(again, woven throughout the book):
-2 Cor. 5:21="...to be SIN(emphasis mine) for us..."
Again, sins(plural) can be forgiven, but sin(singular), cannot be forgiven-it must be judged, condemned. That is, the sin nature, who I was "in Adam,"1 Cor. 15:22 KJV, was not forgiven at Calvary. My then future sins were forgiven........."Christ died for our sins"-plural-emphasis mine-1 Cor. 15:3 KJV.....) but we, as the Adamic "old man,"Romans 6:6 KJV, Eph. 4:22 KJV, Col.3:9 KJV, the source, "root," cause of those sins, was not forgiven. Sin must be judged/condemned, and it was at Calvary.. "For he(God the Father-emphasis mine) hath made him(the only begotten Son of God-emphasis mine) to be sin for us....,"2 Cor. 5:21 KJV, "For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh...,"Romans 8:3 KJV.
Sin-singular. The wages of sin is death...
Thus, while the Saviour was on the cross, it pleased God the Father, for our sakes, to lay our my as-yet-uncommitted sins upon Him,, and His death, by blood, for those sins freed me from the penalty. While the Lord Jesus Christ was on that same cross, God the Father identified(the meaning of the word "baptize") me, in my Adamic life of sin, with His only begotten Son, who was made to be that sin-2 Cor. 5:21 KJV). In Him, "in Christ," I died to sin. Dead. Crucified with Christ. In my death unto sin in the Lord Jesus Christ's death, baptized into His death, I was set freed from all that I was in the "first man Adam,"1 Cor. 15:45 KJV, and was re-created in the "last Adam,"1 Cor. 15:45 KJV, Romans 6:5 KJV, 2 Cor. 5:17 KJV, Eph. 2:10 KJV.
Thus, again, all my sins, plural, were forgiven via the principle of substitution, i.e., "...Christ died for our sins according to the scriptures....,(1 Cor. 15:3, KJV. However, my sin, who I was
"in Adam," was condemned/judged via the principle of identification, i.e., "For he hath made him to be sin for us, who knew no sin....."2 Cor. 5:21 KJV). The Lord Jesus Christ did not die for sin, but for sins. Being made sin, our sin, He was judged, condemned, and crucified in our place.
Thus, the Lord Jesus Christ did not die for sin, singular, but for SINS. Being made sin, our sin, who we are in Adam, by nature kids of disobedience, subject to wrath, He was judged, condemned, and crucified. He, being without sin/sins, died unto sin,out of the realm of sin,having paid the price in full. Thence He was free, qualified, to rise from among the dead into "newness of life"-heavenly, glorified, "new creation life."
I, the natural man in condemned Adam, was not forgiven at the crossfor who I am "in Adam"-"wrong being." My then future sins, plural, were forgiven, but I, as the Adamic "old man", the cause/root/source of those sins, could not be, was not forgiven. Sin must be judged-it was -Romans 8:3 KJV . While the Saviour was on the cross, God the Father laid every last one of my future sins upon the Lord Jesus Christ , and His death for those sins freed me from their penalty. While the Lord Jesus Christ was on that same cross, God the Father baptized me into, identified me, in my Adamic life of sin, my sin nature, with His only begotten Son, who was made to be that sin, 2 Cor. 5:21 KJV. In Him, I died unto sin, was reckoned there. Again, I, the sinful one, was not forgiven for who I am "in Adam"-my sins were forgiven, but not the "old man", the root/source/cause of those sins. I was not forgiven in order to start all over as a "first-Adam" person. No! I was crucified with Christ,Gal. 2:20 KJV, Romans 6:6, KJV), and I died unto sin in Him-dead-sentenced-judgment.....justice carried out, served!. In that death I was reckoned to be severed/ separated from my Adamic life, the source of sin...Romans 7..... The Lord Jesus Christ's death for me redeemed me from the penalty of my sins; my reckoned/imputed death with Him freed me from the condemned/judged Adamic life and its dominion, jurisdiction, control.
The Lord Jesus Christ did not die to make "good"(i.e., no sin nature in men/women argument when born)people "better;" Ney, He died to make spiritually "dead" people, "in Adam," spiritually alive "in Christ."