Purgatory? :evil: :evil:
Keep dreaming! It was done away with 2000 thousand years ago.
Would it be a sin for a Catholic not to believe in the existence of purgatory?
Yes, it would be a mortal sin of heresy for such a Catholic. Read about purgatory in the Catechism of the Catholic Church (numbers 1030-1032). Read about heresy in the same book (number 2089).
We better watch out here, we might get burnt at the stake like the rest of the Christians the Catholic Church has burned.