Romans 5:9-10
Much more then, having now been justified by His blood, we shall be saved from wrath through Him. For if when we were enemies we were reconciled to God through the death of His Son, much more, having been reconciled, we shall be saved by His life.
Simply put, to justify is to declare righteous, to make one right with God. Justification is God’s declaring those who receive Christ to be righteous, based on Christ’s righteousness being imputed to the accounts of those who receive Christ.
2 Corinthians 5:21
For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him.
We are justified, declared righteous, at the moment of our salvation. Justification does not make us righteous, but rather pronounces us righteous. Our righteousness comes from placing our faith in the finished work of Jesus Christ. His sacrifice covers our sin, allowing God to see us as perfect and unblemished. Because as believers we are in Christ, God sees Christ's own righteousness when He looks at us. This meets God's demands for perfection; thus, He declares us righteous—He justifies us.
Romans 3:21-24
God's Righteousness Through Faith
But now the righteousness of God apart from the law is revealed, being witnessed by the Law and the Prophets, even the righteousness of God, through faith in Jesus Christ, to all and on all who believe. For there is no difference; for all have sinned and fall short of the glory of God, being justified freely by His grace through the redemption that is in Christ Jesus,