Therefore, since we have been justified through faith, we have peace with God through our Lord Jesus Christ.” Romans 5:1 NIV
Notice the tense of the verbs. We have been justified. We have peace. Jesus has paid the penalty of our sins. It’s all been done. There's nothing left to do. On the cross, Jesus said, “It is finished.” The work of salvation is complete.
The definition of justified from a theological perspective is that a person is declared free from the penalty of sin on the grounds of Christ's righteousness. You may have heard “justified” described as “just as if I'd never sinned,” which is a good way to remember it. It means we’re made righteous or right in God’s sight. It’s what Jesus has done for us (past tense).
“Everyone who believes in him (Jesus) is made right in God’s sight—something the law of Moses could never do” (Acts 13:39). The Jews weren’t able to obtain justification under the law of Moses, and neither are we. Though many still try by vigorous rule-keeping and performing good works, it won’t work, justification is by faith alone. The lie of the devil is that we must do something to compensate for our sin, by works of some kind to prove our love for God. But it’s not true. We’re no longer under the law and Satan has no basis to accuse us. We prove our love for God by accepting what he has provided for us. Some philosophers try to convince us that it’s by believing something we make it true.
But the Christian says, "It’s true, therefore I believe it, whether I feel like it or not."