The Sin of Romans 5 is unbelief

 Romans 5, speaks of sin. What is the sin in which Paul is talking about? It is the sin of unbelief. Abraham believed God and it was counted as righteousness (Romans 4). Of course, the counting refers not to Abraham's faith but to the object. To the promises of God in Christ. Adam was a man, the proto man, the first man, who stumbled in unbelief. This is the sin by which all men are now born with. The sin of not believing God. But we are saved by the belief of Christ alone. For Christ believed God. By his knowledge we are saved as Isaiah says. It is because we have been justified, by faith we have peace with God - Romans 5:1.

And as it is said, God concludes all in unbelief (11:32). Who believes the report except him who the arm of the Lord has been revealed(John 12:38).

John's first epistle On THE SIN

 in 1 John 3, the sin which John writes is the sin of disbelief of the gospel doctrine that Christ was manifested in the flesh and the hating of those who bring and believe that doctrine. Of course, ultimately the sin John writes against is the sin of unbelief. No questions. Of course, not believing the gospel promise is sin and the result of not believing is that one hates those who believe the gospel. Likewise, those who do believe the gospel should show love towards them that also believe the gospel. Again, what saves is not our love, not even our faith that saves, but what saves is the gospel that is believed.