Mathematics is about logical patterns. A world in which you can be mistaken about such fundamentals as the value of 2 + 2 is not a world where you can put any trust in your logical deductions. As such, if you ever do notice such a slip, I suggest that the cause is likely to be something deeply wrong with you, yourself, and not that you are living in a computer simulation.

The test of any religion is whether cultures believing it tend to thrive and improve the quality of their lives or not. The whole point of the word of God is that following it gives your life "eudaimonia", as Aristotle put it. The Communist religion, for example, failed miserably, and the current secular liberal religion seems to be failing at the "thrive" part. Western flavors of the Christian religion seem to have done pretty well over the last millennium or so, so the move away from it over the last century seems strange. Islam is good at thriving, but seems poor at improving quality of life.

Incidentally, the most fundamental test of Christianity is meant to be belief in the Nicene creed, which is perhaps the best test of whether you believe if "Christianity is true" or not.