I dont really see why teaching christianity (or any other religion) is such a big deal.
There's this thing called "secular government".
It means that no public instituition is allowed to put one religion over or below any other in any way. Teaching one and not others in a public school is effectively free advertising for that one religion, and that's putting one religion over all others.
Which is illegal in any half-decent democracy. So, do you see where the problem is now?
It mostly depends on how it is being given to students.
But teaching it in its own class is not good enough. Because of the reasons above.
Teaching it in a comparative religions or philosophical history class would be okay, and (almost) all others in the thread have agreed so, as long as it was not promoted above the others.