[Ankeseiband]

i think you first need to understand the person you giving the message to. the op said the truth about the Kalima. so if you want to prove the quraan is true to a person that doesnt even believe that Allah exists his going to say that the quraan is made up just as the creater was made up.

so look at the jehovahs witnesses & see how they do it. when they speak to muslims they only speak about the old testiment because alot in there they have in common with us & where we differ they wouldnt mention, so they would speak about Musa AS but not the slaughter of the son of Abraham AS. they would speak about Aadam & the deception of Shaytaan too & about Nuh AS.
when they speak to catholics they wouldnt mention the old testiment but rather stick to the new.

so first sum up who you speaking to & then bring the message over. he might believe in Allah but not Rasulullah SAWS then you dont need to talk about Allah's existance that much. he might be an athiest but loves the example of Muhammad SAWS so then you use that to bring him to find Allah.