Justification itself is defined as being declared righteous before/with God. It is this new legal position before God that is the focal point of the salvation that comes to us through Jesus Christ. The importance of a proper understanding of justification can hardly be overstated. Luther declared justification “the doctrine on which the church stands or falls,” while Calvin described justification as “the main hinge on which religion turns.”
Taking this definition of one being declared righteous before God, justification itself carries the same idea in both the Catholic and Protestant viewpoints; the difference is how one gets to this position of righteousness. In contrast with the infused view of righteousness, which says that God through Christ helps us to become righteous, the scripture can and does give us a clear understanding of how this righteousness comes to us.
Ultimately it is boiled down to a misunderstanding of sanctification and justification that causes this error in the doctrine of the Roman Catholic Church. Rightly separating justification from sanctification clarifies how believers can be declared already righteous (justification) and yet simultaneously being made righteous (sanctification).
Imputed righteousness does not mean that God helps us to become righteous, but that He accounts the rightness of Christ to us by faith alone. (Rom. 4:3) To analogize the idea of infused righteousness, one could say that it is through the work of Christ, that God is over the course of our lives, making us righteous, like cleaning up dirty clothing. We will at times dirty up what God is doing and will need the extra help of the sacraments to keep clean the working of God in our life. These "dirtying" can be small spots that take a little extra cleaning, but can sometimes be of sins of greater magnitude. (i.e. a sin of a grave matter committed with full knowledge and deliberate consent of the sinner) These mortal sins, as they have been called must be confessed and penance paid before death to avoid complete damnation.
The scripture, however, teaches that we are not just seen covered with a layer of unrighteousness or that there is a process of cleaning of unrighteousness being done, but there has been a complete removal of unrighteousness and an imputation of the righteousness of Christ. (2 Cor. 5:21) This justification has entered us into union with Christ that is not just a phase to becoming righteous, but the entire thing. If we were to try and argue that our justification is not complete then we are effectively arguing the work of God is not complete.
Contrary to the infused righteousness position that is carried by the Roman Catholic church, Jesus has declared that His work has been finished, and this includes both the active work imputed to us (John 17:4) and the passive work done on our behalf. (John 19:30) If the infused righteousness view is correct, then Jesus would have been more correct in crying “tag you’re it”, rather than “it is finished”.
For this reason, the doctrine of the imputation of Christ is at the heart of the Gospel; because without it, the Gospel is no Gospel at all.
___________________________________________________________________________________________  Martin Luther, quoted by McGrath, Iustitia Dei, 367.
 Calvin, Institutes, 3.11.1.
 Kenneth Keathley, “The Work of God: Salvation,” in A Theology for the Church, ed. Daniel L. Akin (Nashville: B&H Publishing Group, 2014), 554.
 Ibid. 483.