And then the concept of "grave" "mortal" and "venial" sins seems quite odd to me.
This would be great to discuss in the thread I started about the Nature of Sin
which has garnered little attention.
There are two degrees of sins - mortal and venial. The difference is in how much you turn against God, partially or completely. Grave refers to the magnitude of the offense - for instance, murder is grave matter; lying about your wife's meatloaf (O honey, it was great!!) is not grave matter.
For a sin to be mortal, three conditions must be met:
1) The sin in itself must involve grave matter
2) The sinner must know that it is sinful and gravely sinful
3) The sinner must give full consent of their will in light of that knowledge
What is the effect of mortal sin - it cuts off the life of grace within you. Basically, you tell God to take a hike and get out of your soul! You create a hostile environment within your soul and show God the door.
The only ordinary means for the forgiveness of mortal sin after Baptism is through the Sacrament of Confession.
If you die with any unrepented mortal sins, you go to hell. Why, because you have chosen something else over God. You have chosen to not be with God and so you choose to go where God is not - hell.
Venial sins, while they still damage the soul, don't completely kill the life of grace within you. But, one of the damages venial sins cause is that it makes it easier for you to willingly commit a mortal sin.
These sins can be forgiven outside of Confession (i.e., Eucharist and Anointing of the Sick - and I think maybe Confirmation, but I'm not sure on that one).
Having venial sins on your soul will not send you to hell.
Does this make sense?