"Notice the main point about justification in verse 18: It happens to all who are connected to Christ the same way condemnation happened to those who were connected to Adam. How is that? Adam acted sinfully and because we were connected to him, we were condemned in him. Christ acted righteously and because we are connected to Christ we are justified in Christ. Adam's sin is counted as ours. Christ's "act of righteousness" is counted as ours".
John Piper - "Counted Righteous in Christ: Should We Abandon the Imputation of Christ's Righteousness?" - IVP, Nottingham (2002) - (p107-108).