Seminal questions to ask Christians across the spectrum, and not just fundies:
1) Do you believe that the bible is literally true?
If so, how do you explain its internal contradictions (2 creation accounts in Genesis, 2 genealogies of Jesus - one cannot be of Joseph since he wasn't the father, Jesus' parents not knowing why he stayed in Jerusalem's temple when he was 12 [Luke 2: 42-50]) and historical inaccuracies (no evidence of the flood, no town of Nazareth in the 1st century BC, no record of Herod's slaughter of the infants)?
If not - how do you know which parts are false, which parts must be 'interpreted' and which parts must be taken in 'context'? Can 'context' ever justify acts of misogyny, evil and genocide (Timothy, the Flood, Joshua & Numbers)? If some parts are false, how do you know others aren't as well?
Who's to say that your particular 'interpretation' is correct; with sufficient 'interpretation' anyone can make the Bible say anything they like? If you interpret it according to your god-given religious and ethical sense and instincts, doesn't the fact that everyone has different instincts and senses mean that it is everything to all people? In which case it loses meaning and significance.
2) Do you think all non-Christians/those not of your specific denomination go to hell? If so, how is it just or fair to condemn billions of innocents to hell for all eternity just because they were born in the wrong place at the wrong time? What if you were wrong and, when you died, you went to the Islamic hell? What would you say to Allah?
3) Do you believe that your god is the ultimate arbiter of morality, yet is supremely good? If so, didn't he merely define himself as good, making a mockery of the concepts of morality and goodness? If one defines morality, wouldn't he be amoral rather than supremely good? If it is impossible for your god to not be good, how does the concept of 'good' have any meaning?
To wit: If your god commanded you to kill, rape, pillage and commit genocide (cf the Midanites), would he still be considered good? How would you know that you were under a good god? And if your god were incapable of doing anything evil, would he not have lost his omnipotence?
4) Do you think your mythology unique? If so, why are so many elements shared with previous religions? (Salvation: Mystery pagan religions - Osiris, Eleusinian mysteries, Alleged virgin births of god-men: Romulus, Augustus, Dionysus, Perseus, Baptism: Mithraic mysteries, ablutions, Miracles: Asclepius raising the dead, Resurrection: Dionysius, Osiris)
If you lived in China, could you "invent" chopsticks?
5) Have you ever communicated with, or felt your god or his presence? If so, how would you distinguish your experiences from similar mystical experiences in other religions, people on an acid trip, those who believe they've been abducted by aliens or those who hear voices in their heads claiming to be gods telling them to kill other people?
Deanna Laney, a Texan mother, thought God told her to kill her children, like Abraham and Isaac. The court did not accept her argument. Would you?
6) How does your faith in your religion differ from that of others in theirs? If it is the same, who's to say which religions are correct?
Put another way, if Hermes, the divine Messenger of the Gods, appeared to you with his winged sandals, broad-brimmed hat and Caduceus and told you that you had been chosen to rescue a princess (or prince, if you like) in distress, what would your reaction be?
Would you expect someone from another religion to have a different reaction if the Angel Gabriel appeared to him?
7) If Christianity is such a good religion, why has so much evil been done in its name; why has it failed to rein in all this evil? If the evil was done only by evil or confused people in Christianity's name - despite their faith, wouldn't the same logic apply for the good it has supposedly encouraged?