"If anyone sees his brother commit sin that does not lead to death, he should pray and God will give him life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that he should pray about that. All wrongdoing is sin, and there is sin that does not lead to death" (1 John 5:16-17).
Sin that does not lead to death would be venial, and while it's still sin and should absolutely be avoided, we don't believe venial sin actually cuts us off from sanctifying grace.
So to die in a state of unrepentant mortal sin, one would be lost, but to die in a state of unrepentant venial sin, one would just need to be purified in purgatory before being able to be in the presence of the all-pure and Holy God.
To replace that sanctifying grace, we believe one needs the sacrament of reconciliation, and a penance must be made in reparation for our sins. We believe venial sins are wiped away through prayer, through receiving communion, through graces such as those.
Now something to remember is that no one can unknowingly commit a mortal sin. That is, you can't just accidentally commit one...because one of the requirements is full consent. Here are the three requirements of mortal sin:
1) the sin must have grave matter. (A good look at what constitutes grave matter)
2) it must be committed with full knowledge.
3) it must be committed willingly, with full consent.
Another thing we believe is that, while venial sin does not sever us from God's grace, more and more venial sin makes it easier for us to commit mortal sin. So it's not a good idea to think venial sin isn't a big deal, to say it doesn't matter would be a mistake, because it can lead to mortal sin very easily if we don't watch out.