Smith's Bible Dictionary

Alpha (A), the first letter of the Greek alphabet. With Omega, the last letter, it is used in the Old Testament and in the New to express the eternity of God, as including both the beginning and the end. (Revelation 1:8; Revelation 1:11; Revelation 21:6; Isaiah 41:4; Isaiah 44:6) hence these letters became a favorite symbol of the eternal divinity of our Lord, and were used for this purpose in connection with the cross, or the monogram of Christ (i.e. the first two letters, ch and r, of Christ’s name in Greek). Both Greeks and Hebrews employed the letters of the alphabet as numerals.