Quotes from the Roman Catholic Catechism, taken from the Vatican's website.
The Gravity of Sin: Mortal and Venial Sin
874 To choose deliberately - that is, both knowing it and willing it - something gravely contrary to the divine law and to the ultimate end of man is to commit a mortal sin. This destroys in us the charity without which eternal beatitude is impossible. Unrepented, it brings eternal death.
1875 Venial sin constitutes a moral disorder that is reparable by charity, which it allows to subsist in us.
Justification includes the remission of sins, sanctification, and the renewal of the inner man.
Justification is being seen "right" or "just" in the sight of God. Lutherans understand that justification occurs the moment a person has faith in Christ. This is instantaneous - we are declared just on account of Christ. An acrostic for "grace" is "God's riches at Christ's expense." We are declared just in God's sight on account of Christ's holiness. His holiness is creditted to us and our sinfulness is creditted to him. This is sometimes called, "The Great Exchange".
Lutherans then see that sanctification is distinct and separate from justification. Sanctification begins the moment a person is justified, but unlike justification, sanctification is a process! Instead of being "declared" holy, in sanctification we are "being made" holy. This will never complete this side of heaven. Because sanctification is a process, Christians should be viewed as being both sinful, because we still sin, and saints, because we are declared holy for the sake of Christ's work, death, and resurrection.
Justification remains as long as a Christian has faith in Christ for salvation. Faith, of course, involves repentance. Repentance means to "change one's mind". This is two-fold. First, repentance involves contrition of sin. Secondly, repentance includes, not only turning from sin, but turning to Christ for forgiveness. Many people can have contrition, but not have repentance that leads to salvation!
Roman Catholics, as you see from the catechism quote, believe that sanctification is a part of justification. There isn't a distinction. The process of being made holy is mingled with being seen "just" in God's eyes. This also lends to a system of categorizing sin. Scripture is very clear - sin, any sin, original or actual, brings eternal death. There's no escaping this fact - for anyone! That's a heavy dose of God's law. It shows us our sins, and shows us that we can't live up to his standard. Thus, all sins are mortal sins! All sins warrant and deserve eternal punishment. There should be no distinction between mortal sins and venial sins. This only comes when the gravity of sin is lessoned and our works and our process of becoming holy is inserted into salvation.
Question: Does this mean that a Roman Catholic will ever have certainty of salvation?
Who is the judge that we've progressed enough in the realm of sanctification to be justified before God? Only God. So is there any certainty?
It certainly may seem that I am trying to bash on or bag on Roman Catholic doctrine. I don't do this out of hate. I do this out of extreme concern. Our works should not be mingled into our salvation. I should cite verses to support these statements, instead, I'd like to simply cite two books of the Bible, Romans and Galatians. Read them and see what conclusion a clear reading of Scripture brings, concerning justification, by faith in Christ alone.