Justification is by the faith OF Jesus Christ
Galatians 2:16 Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.
Forgiveness was 2000 years ago too
2 Corinthians 5:19 To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation.
How is it put to an individual's account? By belief in what was accomplished then and there/by the gospel!
1 Corinthians 15:3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
1 Corinthians 15:4 And that he was buried, and that he rose again the third day according to the scriptures:
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Ephesians 3:6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
It is then and only then that the work of Christ is put to our account/"upon all them that believe"!
Romans 3:21 But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets;
Romans 3:22 Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: