Extract from a letter.
"As to 'justification of life,' it is that justification we have as being alive in Christ; that is, it goes beyond mere forgiveness of sins as in the old man which are put away. It is the clearance of all imputation which we have as alive in Christ; but the passage gives us something more specific, it refers to verses 16 and 17.
"Verse 16 is of many offences to justification, which of itself goes further than clearing the conscience of sins. Verse 17 further adds that they who have received abundance of grace, and of the gift of righteousness, will reign in life. This, while based on the clearing, brings us into the new place in life, and reigning in it. Hence we have ' justification of life.' 'By one offence towards all to condemnation.' 'By one complete righteousness towards all men to justification' (v. 18); but then 'in life,' a new life in Christ, not merely, that is, the old sins cleared away negatively, but in the new place by a work of Christ, which God had fully owned. He had finished the work which His Father had given Him to do, and was in virtue of it in a new place as Man in life - life (in us) and justification went together. I do not know if I have made myself plain. It does not go quite so far as the 'in Christ,' but it does identify our justification and a new life in Him." J. N. Darby. 1881.