How do we know that only believers are justified?

Justification is the act of God declaring a sinner to be righteous on account of their faith in Jesus Christ. As stated in Acts 13:39, 'by him all that believe are justified from all things from which ye could not be justified by the law of Moses.' This indicates that it is through faith in Christ, not through law or human effort, that we are made right with God. This assurance of justification is founded on the completed work of Christ and highlights the importance of genuine belief as essential for salvation.