Why -ity and not -ism?

Pointless question. Why is it called Christianity? There’s Judaism, Buddhism, Zoroastrianism, Taoism, Hinduism, why not Christism, or Christianism? (Another exception would be Muslim.)

I don’t know, but it breaks down into Protestantism and Catholicism…
Maybe 2 or more isms combine to make an ity.

[sub]<cricket>…Well I think I’m funny…[/sub]

This ism, that ism, ism, ism, ism/All we are saying
is give peace a chance.

Purity, charity, chasitity, Christianity. Hey, they had to link it somehow.

I thought at first it might be because it ends in an ‘n’ or just a simple consonant. That more or less makes sense. All the others end in vowels, you couldn’t add an ity to them. Christianism, would be the worship of Christians, not of Christ also. Interesting.