The word
sin in Romans 3:25 is ἁμάρτημα (harmatema), and is not the Greek word normally used for sin. It is rarely used in the NT (5x), but it is used quite a few times in the Septuagint (LXX). The
ma suffix means
effect, so the word essentially means
sin effect , or
consequence/penalty of sin. And that is the only meaning that makes sense where is it used in verses like Isaiah 20:2 and Deuteronomy 19:15.
People frequently use Romans 3:25 as a proof text that only one's past sins are forgiven when they come to Christ.
Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; Romans 3:25
But one's past sins are not what this verse is referring to. This is what the phrase
sins that are past in Romans 3:25 refers to:
And for this cause he is the mediator of the new covenant, that by means of death, for the redemption of the transgressions that were under the first covenant, they which are called might receive the promise of eternal inheritance. Hebrews 9:15
So here is a literal translation of Romans 3:25:
[Christ,] whom GOD purposed a propitiatory shelter through faith in his blood into showing his justice through the passing over of the penalties of sin having previously occurred [under the old covenant]. Romans 3:25
2 Peter 1:9 is similar to Romans 3:25:
But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins. 2 Peter 1:9
Like Romans 3:25, the word translated
sins is ἁμάρτημα (hamartema), i.e. sin effect, or consequences of sin. Well, not exactly because someone messed with this verse in some ancient manuscript(s). Codex Vaticanus has the normal word for sin (αμαρτια, hamartia), whereas Codices Sinaiticus and Alexandrinus have ἁμάρτημα (hamartema). Many bibles have αμαρτια (hamartia) because they were translated from Codex Vaticanus.
This is my interpretation based on my belief that ἁμάρτημα (hamartema), i.e. sin effect, is the original word:
Yet to whom these things are not present is blind, closing his eyes, oblivious to receiving cleansing of the former consequences of his sins. 2 Peter 1:9
This agrees with Paul in Romans 3:25. Both Peter and Paul were speaking from a perspective of having lived under the old covenant, and were thus speaking about GOD's passing over of the former consequences of sins committed under that covenant.
The passing over of the consequences of sin in Romans 3:25 is a direct reference, IMO, to the Passover in which the death angel passed over the execution of the consequences of sin (death) in Israelite households covered with lamb's blood. Christ is our Passover. We are justly due judgment for our sins, but GOD in his mercy and justice passes over that judgment (sin effect). That truly is grace.