I am aware of the criteria for mortal sins. However: I sometimes come across different statements of what sins are grave and what is not. For example, the Priest at home says that you go to hell if you miss Sunday Mass. Others say that kissing in a sensual way is a “grave matter” etc.
How do they know? Of course it violates the Commandments and The Preaching on the Mountain etc. but does not that go for traditionally venial sins also (stealing a cookie for example)? Did the Church ever bind certain sins (missing Mass for example) to be mortal?