At what point would a sin become mortal if you learned about it through self-study? Let's say John was baptized as a Catholic as a baby but did not go to Catholic school or receive any religious education at all and his parents did not take him to mass.

As an adult John begins to go to mass by himself.
He begins to read some Catholic books and learned that gossip is a mortal sin. He did not know this up until reading it. Does that mean all his past gossips are considered to be venial sins? If he gossips after reading the material is it then a mortal sin? Or would it only be mortal after a priest has educated him as to why it's a sin?
Mortal sin (from mors = death) forfeits sanctifying grace and is the supernatural death of the soul. St. Augustine defines it as "something said, done or desired contrary to the eternal law."

To be a mortal sin, these conditions must be satisfied:


Knowledge (either natural or supernatural) of the act's sinfulness (offense to God).
Freely performing the sin.
Grave matter


If you didn't know your act was an offense to God at the time of committing it, then it isn't a mortal sin.

Still, in cases of doubt, you should confess it to a priest, who can help judge if it was mortal.