Now it is fair to suggest that in the 70 years which separated Pliny's writings from Celsus' that xtianity conitnued to grow and, less obviously perhaps, diversify into more and more sects which Celsus finds so difficult to sort out. Nonetheless, from his writings I still get the picture that the primary xtian "crime" still relates to their refusal to acknowledge the emperor and the Romans would, especially in the provinces, have been right to consider that seditious behavior.
So you are saying, I think, that Christianity was a crime because Christians wouldn't bow down to the Roman emperor.
And elsewhere, you show that Celsus seems to believe in a form of God ... just not the Christian one.
I would contend, then, that if he had any undertstanding of any God, he would not use the fact that a sect was illegal (particularly for the reasons given above) as part of his argument that their teachings were useless. It's a non sequitur.
That it was illegal is inconvenient ... but it in no way detracts from the value of the teachings of an eternal truth which bears no relation to the politics of the day which come and go with the seasons.
On the one hand, Celsus complains about the secrecy of the Christians, which if he would have completely understood the value of if he was an initiate of the Mysteries. Yet on the other, he complains that Christianity's teachings are open to anyone who wants them - which wouldn't be the case if they were secret. So it's really not clear what his complaint is.
I think Celsus is getting confused here, because he doesn't understand that the Christians, just like the Mystery schools, had one teaching for the masses and another secret one for the initiates. The Mysteries were divided into the Greater Mysteries, which anyone could attend, and the Lesser Mysteries which consisted of those who were creamed off from those who attended the Greater Mysteries because they were considered to be ready.
Thus, in this respect, Christianity at the time of Celsus, was no different to the very Mystery religions which he keeps insisting are so superior.