The text of True Doctrine, written by the pagan philosopher Celsus, comes to us secondhand via Origen's Contra Celsum. If Origen's summary of his opponent's argument is to be trusted, it goes like this (as translated by Raymond Brown):

It was Jesus himself who fabricated the story that he had been born of a virgin. In fact, however, his mother was a poor countrywoman who earned her living by spinning. She had been driven out by her carpenter husband when she was convicted of adultery with a [Roman] soldier named Panthera. She then wandered about and secretly gave birth to Jesus. Later, because he was poor, Jesus hired himself out in Egypt where he became adept in magical powers. Puffed up by these, he claimed for himself the title of God.