The spelling of "most nonbeliever" follows standard English orthography rules. "Most" is spelled M-O-S-T, and "nonbeliever" is spelled N-O-N-B-E-L-I-E-V-E-R. In IPA phonetic transcription, "most" would be transcribed as /moʊst/, and "nonbeliever" as /nɑn.bəˈli.vər/. The word "nonbeliever" is pronounced with the stress on the second syllable, and the "e" in "believer" is pronounced as a short "i" sound. Overall, the spelling and pronunciation of this phrase are straightforward and conform to standard English.
The phrase "most nonbeliever" is not a term with an established etymology. It appears to be a combination of the adjective "most", which indicates the highest degree or extent, and the noun "nonbeliever", referring to someone who does not believe in a particular idea or concept.