Why don't Catholics make the necessary distinction between sanctification and justification?Answer:
They are "virtually interchangeable" as opposed to "always interchangeable" since, though similar, they are speaking about two different aspects of salvation that occur at the same time. "Justification" is usually used to give the reason we can be just before a holy God. In other words, it shows us the "justifiable" reason God can saves us, that is, because of Christ's atonement He can give grace to the human soul.
"Sanctification" is usually used to show the state of grace, that is, whenever you receive saving grace your are sanctified. You are in a state of grace, which if you died, would allow you to go to heaven.
But since in the Catholic faith both justification and sanctification occur simultaneously and continuously, and perform the same function, that is, to provide grace in the soul for salvation, then they are synonymous and virtually interchangeable, which is why the NT often interchanges them. The NT separates justification and sanctification usually when it is trying to focus on one of the two aspects I mentioned above, i.e., the reason for, as opposed to the state of, grace.
Catholic Apologetics International
November 7, 2001 A.D.