Quote from Cutten:
You look at the available evidence, and make your best judgement.
My criteria is that the more and better quality evidence there is for something, the more I will take it on board as a possibility. Also, the plausibility of the idea must be taken into account. Implausible things with no evidence to back them up are occasionally true or real, but that's not the way to bet - most times they are just BS. E.g. the tooth fairy, the Yeti etc.
1000 years ago, AFAIK there was no evidence for cosmic rays existing, based on human knowledge at the time. If that is true then the odds were against them existing.
Remember that scepticism is a pragmatic technique for trying, under conditions of limited knowledge, to assess which out of countless theories and propositions are most likely to be true. In 1009 AD, there were doubtless many theories with little or no supporting evidence, almost all of which would turn out to be totally wrong. That a few of these unsupported theories turned out to be right simply means that - at the time - they were a lucky guess. Guessing and relying on luck has historically not proven to be a good way of judging whether theories are right or wrong. Favouring theories based on their supporting evidence, on the other hand, has proven to be pretty successful and that is why it is the basis of scientific thinking.
My question in return would be "How do you know God exists?". What criteria are you using to form this belief, how would you know if you are wrong etc.