In this passage Paul begins sharing the Good News: We are sinners, but we have been saved! God has made us righteous and justified apart from the Law which we could never follow. Our righteous standing before God is a gift received by faith in Jesus. It's a gift of God's grace.
Our justification before God wasn't just a one-time thing (and then we must live the rest of our lives perfectly). The Greek verb tense Paul uses here shows that we are continually justified. Even though we continue to sin as we grow in Christ, we remain in a state of being continually justified. This allows us to grow without the burdens of guilt and condemnation that can hold us back from true, sustained growth.