Ramadan. Why do Muslims call Atheists to Islam?

muslims believe that all the Prophets will be asked about the conveyance of teachings to the people of their age and since there would be no Prophet after Muhammad (SAW), muslims have to carry the message and they will be held responsible for not conveying the message of Almighty to the rest of human being. Muslims are responsible only to convey and NOT to compell anyone. Everyone is responsible for his own deeds.