The Catholic Church teaches that there are two categories of sin: mortal sin and venial sin. Venial sins hurt our relationship with God, but do not sever it. Mortal sins, however, concern grave matter, and an act of mortal sin will cut us off from God - from heaven - unless we repent.
There are three conditions necessary for a sin to be mortal:
1) It must concern grave matter, such as the Ten Commandments.
2) It must be performed with full knowledge that the act is wrong
3) It must be performed with full consent - the person freely chooses to commit it (not done by accident or due to physical coercion)
- Understanding Mortal Sin (beginningcatholic.com)
- Mortal versus Venial Sin - By Colin B. Donovan (ewtn.com)
- Sin - From the Catholic Encoclopedia (newadvent.org)
- Catechism of the Catholic Church - Sin - Part IV is about mortal and venial sins (vatican.va)
- Why Mortal and Venial Sins - A former Evangelical tells how the Bible led him to believe in different degrees of sin; by Mark P. Shea (catholic.com)
- The Truth Hurts: Mortal Sin - By Marco Fallon (bringyou.to)
- Salvation - See "Scripture Part IV" and "Tradition Part II" regarding losing salvation (scripturecatholic.com)
- Mortal Sin - Brief introduction and quotes by the early Church Fathers (catholic.com)
- Ignorance - Invincible and Vincible - And how it affects the gravity of sin; by Jimmy Akin (catholic.com)
- Why Is It a Mortal Sin to Miss Mass? [Deliberately] - By Fr. Ray Ryland (catholic.com)