Romans 5:13

‘For until the law sin was in the world, but sin is not imputed when there is no law.'

Sin was in the world from the moment of Adam's fall. This happened before the Law came into the world, the Law which made sin apparent for what it was. As a consequence men sinned, but as there was no God-given Law by which they could be demonstrated as blameworthy, man could not pass judgment on men. Judgment was very much left in the hands of God, for man was in no position to pronounce on what was sin. Man was unable to ‘impute sin'. Once, however, the Law was there man could impute sin. In other words he was able to demonstrate that it was blameworthy in the eyes of God and could therefore act as judge on God's behalf. But he had not been able to do that before. We can consider how Cain's sin was brought home to him by God, not by Adam (Genesis 4).

We cannot really suggest that Paul was saying that  God  could not impute sin, for he would have been very much aware that God had clearly imputed it to Cain (Genesis 4:7), and had equally clearly imputed it to mankind when He destroyed them by the Flood (Genesis 6-9). Consider also the sins of Sodom and Gomorrah which were clearly imputed to them (Genesis 18-19). In each case God called them to account on the basis of what He and of what they knew to be wrong. How else could He have been seen as the Judge of all the earth Who did what was right (Genesis 18:25)? It was thus man who, in so far as it was so, was left in the dark as to what was sin. And even then he had received various directions from God (e.g. Genesis 9:6; Genesis 18:19; Genesis 26:5), so that he knew of some things which were displeasing in God's eyes. Indeed for Paul to suggest that God would not impute sin would be partly to negate his earlier argument about the law written in men's hearts. The point being made here, therefore, is not that God could not impute sin, but that men were unable to point the finger at each other, and sentence each other on the basis of it. It was they who were unable to identify sin and bring it into condemnation.

The importance of this for Paul's argument lies in the fact that a Jew might argue against all being seen as having sinned on the basis that sin could not be imputed before the giving of the Law. ‘Nevertheless,' says Paul, ‘that all sinned is demonstrated by the fact that all died.'