To make a way for us to be saved, God sent Christ to live a perfect divine-human life and die an obedient death. In this way Christ became both the substitute punishment for our sins (Matthew 26:18; I Corinthians 15:3; I Peter 3:18) and the substitute performer of our righteousness (Romans 5:19, 10:4; II Corinthians 5:21; Philippians 3:9). Therefore, in the courtroom of God, my guilt for sin is removed by Christ’s blood (Ephesians 1:7) and my title to heaven is provided by Christ’s obedience (Romans 5:19). I am declared just – freed from the punishment of sin and now possessing a title to heaven. This is what we mean by justification.
– Piper, John; When I Don’t Desire God: How to Fight for Joy; copyright 2004; Crossway Books; Wheaton, Il.; p. 83