The word Semitic did not originate as a term any of the peoples it describes used for themselves. Eighteenth century European linguists, working from the genealogy in Genesis chapter 10 that traces various ancient peoples to Noah's three sons, Shem, Ham and Japheth, adopted Shem's name as a convenient label for the family of related languages that included Hebrew, Arabic, Aramaic and the older languages of ancient Mesopotamia and the Levant. The classification was linguistic from the start, grouping languages by shared grammar and vocabulary rather than grouping peoples by shared religion, ethnicity or self-identification, and it has stayed that way in serious scholarship since, however loosely the term has sometimes been used outside it.
That linguistic definition produces a strikingly diverse population when applied to actual speakers. Arabic alone is spoken natively by hundreds of millions of people across the Middle East and North Africa, the overwhelming majority Muslim but including significant Christian populations in countries such as Lebanon, Egypt and Iraq. Hebrew, revived as a modern spoken language in the twentieth century after centuries as a liturgical language, is spoken by the citizens of Israel. Aramaic, once the lingua franca of much of the ancient Near East and the language Jesus is generally understood to have spoken day to day, survives today only in small, scattered Christian and Mandaean communities. And Ethiosemitic languages including Amharic and Tigrinya are spoken by tens of millions of people in Ethiopia and Eritrea, a population as religiously mixed between Christianity and Islam as any other Semitic-language-speaking region.