Faith, your name says it all. There are certain things that we can say for certain to be true and factual. Mathmatical proofs are impossible to argue with. ie: 1+1=2. If someone wants evidence as to why this is so then it can be expressed in many ways. 1 apple, and then get another apple then you have 2 apples. If something can be shown to occur the same way over and over like the reaction of 1 Hydrogens plus 2 Oxygen make water then it can be expressed mathmatically as well. Or shown physically in a lab.
Now to my point. If I say that I know for a fact that God exist.
Then I must base this knowlege on facts. As a truth It must be somehow expressed in a way that is universally accepted to anyone who would care to inquire.
The problem is that some people do not distinquish what they KNOW to be true from they're beliefs. As far as they are concerned it is not a matter of discussion. There beliefs are beyound reproach. Which is fine. But understand you are dealing with people who do not hold those same beliefs to the same standard that you do.