If you could tell a person who tries to convert you to their religion one thing about why you believe the way you do, what would it be?
A question rather: Where was your god during slavery, Holocaust, 9/11, My Lai, Jonestown, Opelousas, Sandy Hook, Oklahoma City, Vegas, Pulse, Colfax, Syria, La Bestia, Khmer Rouge, Idi Amin?
There, being all omnipotent while watching his creations get slaughtered, some in his name, because everything happens for a reason and was his will? Riiiight Isn't it easier just knowing some are good, some bad and shit just happens?
I would like to think that I would try polite logic and rational thought but that has failed so many times in the past that it would take blunt talk to even register with them. I think that I would just chuckle and silently walk away. At least I would Not be a vitriolic hateful, stereotypical memory that they expect.
My favorite response is to explain you have a high standard for truth as you demand evidence to support any truth claim.
The foundation of all religions is faith. Faith is (by definition) the belief without evidence asserted as "Truth".
As you actually care about what is real or true, and their assertion is based on faith asserted as truth (which I personally find dishonest), I have no interest in wasting my time listening to any (faith based) assertions.
Side option: If religions were actually based on reality or truth, there would be no need for faith.
Would you take the word of a dirty, smelly, mentally ill hermit walking around town with a placard stating that the world is going to end or a cogent, intelligent scientist who says the world is older than 5000 years?