God imputes righteousness to you through His Son. What does "impute" mean? To put on your account. At the moment you believe on the Lord Jesus Christ, God makes you righteous. Romans 4:8 says, And this is without you doing any work to get it! Also, the Bible says, "Blessed is the man to whom the Lord will not impute sin." A lady I know was asked one day what it felt like to be saved and she replied, "It feels as though I am standing in Jesus' shoes and He is standing in mine." I don't think a scholar or theologian could have said it any better.
"For He hath made Him to be sin for us, who knew no sin; that we might be made the righteousness of God in Him" (2 Corinthians 5:21). Jesus paid it all on the cross of Calvary. He became sin for us, so that HE could pay the sin debt for me and you. Thank God HE did that for us.