They didn't get them from thin air. They got them from Christians. This amounts to a no true Scotsman defense--all the things all those other Christians say, they aren't true Christianity.

You make an excellent point. There are a number of things being proposed by groups that call themselves Christian, often in the honest belief that they are right to propose such things (and to do so enthusiastically), which I nonetheless find myself in firm disagreement with. (For example, creationism).

To avoid the fallacy, then, and to deal with such contradictions, I shall define more narrowly what I consider "true Christianity", and I shall define it as Roman Catholicism (or something sufficiently close to it).

Christianity says you should do X. If you are only required to follow Christianity to your best understanding to be saved, and you don't understand Christianity as requiring X, you don't have to do X to be saved. But once you really understand that Christianity requires you to do X, then all of a sudden you better do X. Following it to the best of your understanding means that the more you understand, the more you have to do.

And I'm sure you can think of plenty of things which Christianity tells you to do. It's not as if examples are particularly scarce.

One example of X that I can think of, off the top of my head, is "going to Church on Sundays and Holy Days of Obligation".

It is true that one who does want to be a good Christian will need to go to Church, while one who is ignorant will also be ignorant of that requirement. Hmmmm. So you have a clear point, there.

The way God is described by Christians looks just like malevolence. If God really saves people who follow Christianity to the best of their understanding, without loopholes like "maybe he will save them but maybe he won't so becoming more Christian is a safer bet", Christians wouldn't proselytize.

I think that one reasonable analogy is that it's a bit like writing an exam at university. Sure, you can self-study and still ace the test, but your odds are a lot better if you attend the lectures. And trying to invite others to attend the lectures improves their odds of passing, as well.