Surely purgatory/praying for the dead is all rather bound up together, MaryBS? (Gosh, I know why you have BS in your name, but it does make me feel like I'm being very rude about your posts when I name you )
The Catholic Catechism now teaches that non-Catholic Christians are 'separated brethren', and part opf the church but without the 'fullness' of truth.
I believe it also teaches that God has mercy on anyone of another faith who follows that faith in sincerity. I know Mother Theresa believed that.
I'm not sure whether Trent is still in force in the bit about placing a curse on those who believe in salvation by faith apart from the works of the law. It all gets a bit confusing at this point (if it hasn't got that way loooong before...)
To get into heaven, you must be in a state of grace, but if you've still got residual venial sin knocking about, you need that paid for. Saints are them as have no mortal or venial sin when they die.
By performing penances and gaining indulgences, you can add to the treasury of merit which Christ put the biggest deposit in, basically, and it's this merit that gets applied to the souls in purgatory.
(This is how I understand it all, and I've had a chat with a Catholic pal this morning on the phone )