Well 1st you aren't going to make christians doubt their beliefs unless they already have a doubt as it's called "faith" for a reason.

any case my go to question is the same question I asked myself at age 15 when the priest told me before I was to make my "confirmation" that this wasn't like all prior rites, that I had to be sure I was ready... ala he asked me to actually think for the 1st time. the question was....

if this universe couldn't have come into existence by itself so therefore it had to have a "creator" (as I learn may years later is their "clockmaker" argument) then who/what created the "creator"? so if one believes in a creator for this reason then logically an even more powerful entity had to create that creator, and that entity had to have a creator, and so on for infinity.

since they won't buy logical reasoning then you move on to the next question.... why do they believe specifically in their christian god? the #1 answer, though they may skirt the question, is that they were lucky enough to have been born to parents who they inherited their religion from. heaven forbid they were born in another country where the predominate religion wasn't the same as theirs. now they may skirt the question as I said by saying they've done their own "research" and re-affirmed their belief that an up until that time a god decided to make themselves known and befriended a relatively small tribe in the middle east, and luckily some centuries later someone would fuse pagan mystery religions full of dying and resurrecting god-men born to virgins over lots of centuries prior and try to give it more street cred by wedding it to a local but well established religion.

Street Epistemology. There are good examples on Youtube. There are hundreds of vids but this is a good explanation. It's based on the Socratic method.