As noted in the discussion under 1 Nephi 1:19, the editions have typically capitalized common nouns usually reserved for deity, even when the referent is not actually deity (as here in the Words of Mormon 1:15). Interestingly, both the 1837 and 1840 editions (which were edited by Joseph Smith) set the lowercase spelling christs for this passage, but beginning with the 1849 LDS edition, all printed editions have had the uppercase spelling Christs. The RLDS textual tradition, beginning with the 1858 Wright edition, has also had Christs. The critical text will maintain the capitalization of words like Messiah, Savior, and Christ, even when they refer to false claims of deity (as here in the Words of Mormon 1:15).
Elsewhere there are five instances where nonbelievers refer to their lack of belief in “a Christ”:
The printed editions have capitalized all these instances of Christ.
Summary: Retain the capitalization of Christ even when it refers to false Christs (here in the Words of Mormon 1:15).