The Catechism of the Catholic Church teaches:
To understand this doctrine and practice of the Church, it is necessary to understand that sin has a double consequence. Grave sin deprives us of communion with God and therefore makes us incapable of eternal life, the privation of which is called the "eternal punishment" of sin. On the other hand every sin, even venial, entails an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory. This purification frees one from what is called the "temporal punishment" of sin. These two punishments must not be conceived of as a kind of vengeance inflicted by God from without, but as following from the very nature of sin. A conversion which proceeds from a fervent charity can attain the complete purification of the sinner in such a way that no punishment would remain.
It is often the assertion of our separated brothers and sisters that Jesus' Death on the Cross and their acceptance of Him as their personal Lord and Savior means all sin has been conquered. They cannot lose their salvation., they claim. They cannot do anything to merit their salvation and so their works mean nothing - just as their sin, if they commit any - cannot keep them from heaven.
Sometimes, especially if you talk with someone who has some actual theology study behind them, they will assert that if someone does commit a sin after accepting Jesus into their heart then it means they really didn't accept Jesus into their heart...which gets very confusing and doesn't make a lot of sense.
The Catholic understanding of sin is not only reasonable it is true.
One is saved through the merits of Jesus Christ's sacrifice on the cross. Nothing else can open the gates of heaven. Our own righteousness does not accomplish this and that is evident by the fact that our elder brothers and sisters in the Faith did not enter into heaven upon their death. In fact, Jesus had to descend to them and lead them home. Catholics know, believe and understand that anything we do is not about saving ourselves or mankind.
We also understand that we have to work out our salvation, and do so with respect for God, His Church and our own concupiscence (Phil 2:12). We know that we have a goal to attain, a goal of our Faith and that goal is salvation (1 Pet 1:9). We cannot be so arrogant as to think we cannot, willfully and intentionally, leap out of the Hand that holds us and that Our Father will not respect that decision. God wants us to be saved - He doesn't make us saved against our own free will.
Jesus promised me (Mt 10:22) that if I endure to the end I will be saved. He made it pretty clear that I have to be willing to lose my own life for His sake if I am wanting that life - the eternal life - to be one spent with Him (Mk 8:35). He also made it pretty clear that what I do and how I do it is going to matter a lot when I stand before Him to receive the General Judgment. St Paul told me flat out, I am created in Christ Jesus for good works (Eph 2:8-10) and if I don't do that I may be one of those people who do not get to enter heaven (Mt 7:21).
None of this ever mattered to me as much as it does now, not because I am a better Catholic now but because I see so many people today making foolish and childish choices. I know I did the same but now, because I have lost so many loved ones, I have a much better understanding that any day, any hour, any minute could be my last. I could be standing in front of Jesus for my Particular Judgment this afternoon. I could be standing there 35 years from now. Either way, I have to make sure I am doing my part to assure my standing in the Book of Life.
Please keep those who have strayed from the path of Truth. My heart goes out to them; I will not chase after people but it always makes me sad when they walk away.