Some Jews consider Jesus to have been an ordinary man and write his name like that of any other man. Some question whether or not he even existed, possibly being a myth borrowed from similar stories. Others ascribe to him the status of a "deity worshipped by others," whose name Jews should not pronounce. Many extend this ban to the written form. Some write "Xianity" as a simple shorthand, like "Xmas," while others prefer not to write "Christianity" lest it appear that they consider Jesus to have been the Messiah.
Note that the shorthands "Xianity" and "Xmas" do not derive from attempting to "blot out" the Jesus's name; rather, they arose because the first letter of the Christ in greek (Christos) is a Chi, which looks like an "X". In fact, the shorthand is used by many Christians.
The possible halachic problem with writing Christ derives from the fact that "christos" is the Greek word for Messiah/moshiach.
Hence some argue that writing the name Christ in full tacitly acknowledges (G-d forbid) that Jesus was the Messiah.