The 1906 LDS edition added the indefinite article a before God; but because this edition never served as the copytext for any subsequent LDS edition, this error was never transmitted. The phrase a God is found elsewhere in the text, but only in existential expressions (such as whether there is a God or not). But this is not the issue here in Alma’s discourse, so a God does not really work in this passage. Moreover, the following verse supports the use of God without a: “but God ceaseth not to be God” (not “but God ceaseth not to be a God”).
Summary: Maintain God without any determiner in Alma 42:22: “and God would cease to be God”.