the prophet (saas) was always very patient and understanding with those who denied the existence of allah, the qur'an, and his own prophethood. he also made it clear that he would treat them with full justice and not enter into pointless arguments with them. what he said, in accordance with the command of allah, is recounted in the qur'an:

so call and go straight as you have been ordered to. do not follow their whims and desires but say, "i believe in a book sent down by allah and i am ordered to be just between you. allah is our lord and your lord. we have our actions and you have your actions. there is no debate between us and you. allah will gather us all together. he is our final destination." (surat ash-shura: 15)