I think that G-d could have hardened the hearts of the original followers of JC so that they would have followed him after he died, even if they should have stopped. Or they might have been influenced by an evil spirit or lying spirit.

The evil spirit from G-d affected Saul in 1 Sam 16:14-16, and 23, and in 1 Sam 18:10, and 1 Sam 19:9.

The lying spirit was sent to prophets in 1 king 22. Maybe the original followers of JC had something like that that encouraged them to be false prophets.

2 Kings 19:7 in Hebrew mentions a spirit put on an Assyrian for some reason. It could just mean a feeling, I guess, and not an evil spirit.

I hope that you feel enlightened by these quotes.

You suggested that I should enlighten you in your response.

You asked me why would G-d let an evil spirit or lying spirit go against G-d. Ask G-d. I am just pointing out quotes. I assume G-d has an evil spirit and a lying spirit for some purpose because the Tanakh mentions them. I assume there are also evil angels for some purpose.
Sincerely,
Manwithdream