What constitutes a mortal sin?

Answer 1

Catholic Teaching:

A mortal sin, according to the Catholic Church, is a sin of a sufficiently serious nature such that dying with one of them on your soul results in instant passage to hell.

This contrasts with venial sin, which is serious enough to keep you from heaven, but not serious enough to merit an eternity in hell. Venial sin on your soul at death (except, perhaps if one dies as a martyr) would cause you to go to purgatory, where your soul is purified before you can gain entrance into heaven. Purgatory is very much like hell experientially, except that it is not permanent.

These ideas are not considered tenable by groups that hold to "Salvation by Faith" theologies and the denominations that come out of them.

Mortal sins include: (Some of these can change from time to time)

Missing Mass on Sunday

Eating meat on any Friday (until modified by Vatican II)

Missing Mass on a Holy Day

Failing to perform one's "Easter Duty"; that is, receive Holy Communion at least once per year

Answer

According to the beliefs of Roman Catholicism, a mortal sin is a sin that, unless confessed and absolved (or at least sacramental confession is willed if not available), condemns a person's soul to Hell after death.

A grievous matter, sufficient reflection, and full consent of the will, are the conditions that make a mortal sin.

Protestant Teaching:

The distinction between mortal and venial sin is entirely an invention of the Catholic Church. The Bible teaches that 'the wages of sin is death.' It makes no distinction whatever. Sin is sin.

Catholic Church

I would like to clarify: having a mortal sin on you WEAKENS your chance to heaven, not destroys it. If you are truly sorry that you committed that sin and have made it clear that you have, you increase your chance into heaven. Also, i'd like to say, its not our place to judge who goes to heaven or hell. Just saying.