If Tacitus was the ONLY document on which our faith hinged, you might be right. However, the fact of the matter is that Christians base their belief on 20 or more things making a CUMULATIVE case. So if even 1/20th or even 5/20ths have problems, that’s not enough to sway. It’s like a court case where not everything is figured out, but there’s enough to convict beyond a reasonable doubt. That’s true of almost all cases.
Now if you DO want one thing to shake up Christians, disprove the resurrection. But beware, almost all who try it end up becoming Christians. (C.S. Lewis, Josh McDowell, Jim Warner Wallace, Gary Habermas, Frank Morison, Sean McDowell, etc.)
The claim of a god/man resurrection isn’t special or unique to Christianity. Dying and resurrecting gods were very common in the ancient near east. Osiris, Adonis, Innana, etc. Each were created by their cultures and none of them were real. Trying to disprove that the resurrection you believe in is false is like trying to disprove that Dorothy never made it to the Emerald City.