How do we know justification is true for believers?

Justification is a key doctrine in Reformed theology, asserting that believers are declared righteous based solely on their faith in Jesus Christ (Romans 5:1). This truth is not reliant on human effort or merit but rests on the finished work of Christ, who lived a perfect life and fulfilled the law in our place. As Paul emphasizes in the same chapter, it is by faith that we receive this justification through grace. Therefore, every believer can stand before God as if they have never sinned, simply because Jesus Christ, our substitute, took upon Himself the punishment we deserved.