[Kris'-en-dum ] generally means all Christian countries; but Shakespeare uses it for baptism, or “Christian citizenship.” Thus, in King John, the young prince says:-
By my christendom! So I were out of prison and kept sheep, I should be merry as the day is long.
Act iv sc. 1.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Christendom from Infoplease: