The short one
  B:     Do you know that P = Q  ?
  NB:  Can you prove it?
B:     Why don't you disprove it?
NB:     But you said it was true, so prove it!
  B:       Ha ha - prove it wrong friend or else accept that it is true
The classic one
 B:                I think X = Y
NB:            But we have absolutely no explanation or evidence to support this
  B:               It has been divinely ordained
NB:           But even in a court of law, they require evidence
B:              Friends, we are talking about something much much above these trivial institutions.
                  How dare you compare HIS wisdom with that of a mere judge in a court of lawThe most popular one
B:       Don't you feel the presence of the Almighty, all around you??
 NB:   I tried my best but I really do not experience anything out of normal.
  B:      There must be something wrong with you. Only those who have true faith & surrender to the lord are able to feel HIS presence.
About omnipotence
B:       Don't you know that GOD is omnipotent and infinitely powerful? 
NB:    Oh! Does that mean GOD has decided what is going to happen in your future?
B:       Of course! We live only according to HIS will.
NB:   But if he has decided your future, this means that he cannot change it - so how can you call
        HIM infinitely powerful?  B:      No HE can change it if he wants to
NB:   But that means HE is not sure whether HE is going to change it or not.
B:      Oh these things are beyond your puny intelligence. Let's talk of something else.
About omnipresence
NB:  Where is HE?
B:     Oh HE exists everywhere
  NB:   Then why do you want to go to Tirupati to worship Balaji - you can do so right here in the Balaji temple in our town.
  B:      No but Tirupati is a  more auspicious location
NB:   But  I thought you just said that HE is everywhere. Then why is his temple at Tirupati more auspicious than at Nagpur?
B:      Oh you don't understand what spirituality is all about.
NB:    What has this got to do with spirituality?
B:       There is no point in talking to an illiterate like you.
  About our purpose on earth
  NB:    Do you think all of us have a pre determined purpose of getting born on earth?
  B:       Of course! HE has sent us on earth with a specific purpose
NB:    What could have been HIS purpose in case of the people who got killed in the tsunami
B:      Oh HE works in HIS own mysterious ways.
About prayers
  NB:     Why do you pray so often?
B:        To pay my respects, ask HIM for success and happiness & to reduce my hardships
  NB:     But HE has already decided to give you those hardships so why will he change HIS mind  
        now just because you are pleading through your prayers?
  B:       But I must regularly pay HIM my respects by worshiping HIM?
NB:    Does HE demand worship and prayers? And will HE listen to only those who pray? 
B:        Everyone is equal in HIS eyes.
NB:    So why pray? If your intentions are good and you are leading a good moral life, HE is  
       watching you and will automatically reward you
B:       I am not going to continue this pointless discussion.
NB: Ok, how are you sure that HE is listening to your prayers?
B:    Oh, he has HIS own ways of letting me know about that.
NB: But then if I pray how will I be sure that my prayers have reached HIM? I need some
        advice from you because you have been regularly praying to HIM.
B:    You start praying first and you will start receiving HIS signals.
The bizarre one
NB: How does HE decide when two competing devotees want their wishes fulfilled?
NB:  If that is true, how are  politicians and their cronies able to corner most government
       contracts?
The clincher
NB:   Do you really think that HE will look after you?
B  :    Of course HE will. He is powerful enough to take care of all his creations
NB  :  If he is really all that powerful, why do we lock temples. Can't HE look after HIMSELF?