The whole point of the gospel is that the minute I embrace Jesus Christ, all that Christ has done is applied to me. All that He is becomes mine, including His righteousness. Luther’s phrase “at the same time just and sinner” means that, at the very instant that I believe, I am just by virtue of the imputation of Christ’s righteousness. It is Christ’s righteousness that makes me just. His death has taken care of the punishment I deserve. His life has made possible my eternal reward. There it is. My justice is all tied up in Christ. And yet at the same time, in and of myself I am a sinner. It is sinners who are saved by the atonement. That is the glory of the gospel and of the cross. The Bible tells us that the only way we can have the righteousness and the merit of Christ transferred to our account is by faith. We cannot earn it. We cannot deserve it. We cannot merit it. We can only trust in it and cling to it.
“In him we have redemption through his blood, the forgiveness of our trespasses, according to the riches of his grace, which he lavished upon us in all wisdom and insight” (Ephesians 1:7-8)