With the late Jews and the Greeks the letters of the alphabet were used to denote numbers; a name then could be given enigmatically in the sum of the numbers denoted by its several letters. Thus in Gen. Gen. 14:14 the number 318 was taken by the rabbis to denote Eliezer; the numbers denoted by the respective letters of that name added together form this sum. The Christian Sibylline I. 324 ff. uses 888 for the name ʼΙησοῦς [Iēsous] , Jesus.1
See commentary on Revelation 13:18
1 Isbon T. Beckwith, The Apocalypse of John (Eugene, OR: Wipf and Stock Publishers, 2001), 403.