"One day you will know the truth." At the very least, this is somewhere between dismissive and insulting. Obviously, the speaker is asserting that you do not yet know the truth. Or Truth, more likely. Truth about what? This is often left vague. Taken at face value, this could mean a nearly infinite number of things:
- Why am I ticklish, yet unable to tickle myself?
- What is that notch in my upper lip under my nose called?
- Does Bill Nye wear clip-ons or real bow ties?
- If the latter, does he tie them himself, or get help?
- If he gets help, from whom?
Of course none of these is what a religious person has in mind when they repeat this chestnut to an atheist. The truth in the believers' minds is the truth and reality of their own religious position. They are expressing a smug expectation that after healing whatever ails them, the atheist will return to the healthy position of belief.
It does raise a larger question of, how do we know what is true? Without going too far into the immense philosophical field of epistemology, I will say that I believe that the highest-quality search for the true nature of the world is going on wherever the scientific method is being most carefully applied. The safeguards of the scientific method to root out subjectivity and logical fallacy are simply ignored in the halls of theology, where personal experience governs as fact and dogma plays the role of theory.
This is #4 of a series covering the top ten goofy things religious people say to atheists.