it's not a sin not to go to church, but God said that we must not forget to assemble ourselves or come together to hear His word, to encourage one another, to receive strength. It's like this people get upset when you talk about God and what He requires, but I bet you make sure you attend parties and gatherings, if you can do that for mere men, why can't we do the simple things for the one who holds are very breath in HIS HANDS.
Hmmm. I don't think it is a sin, its not a commandment that can be broken, or one of the seven deadlys. More, in my view, just a way of showing respect, keeping in the sight of God and all that. And Christianity, Catholicism, is a very social orientated type religion, it seems good to ardent followers to get together, in a religious environment.
The little known 10th level of hell, where people who don't go to Church go? Hehehe.
Somehow though, I don't see why a person would be punished in such a way. Don't go to Church? In with the traitors and murders and the brimstone then.... it would be a bit like sending somebody to the gas chamber for littering. And thats if its a crime at all.