"letters of a language arranged in customary order," 1570s, from Late Latin alphabetum (Tertullian), from Greek alphabetos, from alpha + beta. Attested from early 15c. in a sense "learning or lore acquired through reading." Words for it in Old English included stæfræw, literally "row of letters," stæfrof "array of letters," and compare ABC.
It was a wise though a lazy cleric whom Luther mentions in his "Table Talk,"—the monk who, instead of reciting his breviary, used to run over the alphabet and then say, "O my God, take this alphabet, and put it together how you will." [William S. Walsh, "Handy-Book of Literary Curiosities," 1892]
Alphabet soup is attested by 1907.