How do you know if a sin is mortal or venial?

I understand the purpose of Purgatory and do not deny its necessity and existence. I am familiar with the text in Matthew that states the only unforgivable sin is that of denying the Holy Spirit. It is with that in mind that I become confused with mortal sin. I have heard on the broadcast that some mortal sin (such as masturbation) can be considered venial if it is being done in the context of being addicted. My question is if you have committed one of these mortal sins and have not had a chance for confession yet, how is it we know this will condemn one to hell and not to purgatory for additional purging? The clear fact we refer to it as mortal does definitionally mean we as Catholics believe it is capable of removing us as “God’s Friend” thus not allowing us to make it to purgatory.

These links should explain the Roman Catholic Church’s teachings on mortal and venial sins. If you have any further questions or concerns that are not answered by these links, please contact Catholic Answers directly.

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