There are two types of sin: mortal and venial.
A particular sin is either mortal or venial, depending on the severity. (For instance, stealing one dollar from a rich man would probably be a venial sin.)
The pope is Jesus' representative on Earth.
Sources of doctrine include:
Certain papal declarations
Bishops in conjunction with the pope
Apocrypha (some additions to the Bible)
Catholic Church Tradition
Catholic interpretation of the Bible
Anyone who denies the authority of the pope despises the one who appointed him (i.e., Christ) and therefore despises the one who sent Christ, (i.e., God the Father).
3 True/False Questions
Purgatory is totally unscriptural. Christ's sacrifice on the Cross was the only offering necessary and the only offering sufficient to provide salvation → CATHOLIC
There are two types of punishment after death: temporal (temporary) and eternal. If a person dies with just one "mortal" sin on his soul he will be condemned to Hell for eternity. If he dies with only "venial" sins on his soul he will be sent to Purgatory, perhaps for millions of years.
Purgatory is exactly like Hell except that it doesn't last forever. Eventually, the person will be released and enter heaven.
This is totally unscriptural. → CATHOLIC
The pope speaks for all Christians.
The pope speaks for no one. → CATHOLIC
Catholic priests have been given the power to forgive sins, acting as representatives of the Holy Spirit.