Blessed be the Lord, thy God, who hath give us the Signs, who is the Christos, the Alpha and the Omega. 

This symbol, from the Equinox I:6(?), represents the Chi-Rho, ⳩, an abreviation using the first two letters of ΧΡΙΣΤΟΣ, Christos. This symbol is actually older than Christ, previously representing the word ΧΡΗΣΤΌΝ, meaning “good” or “useful.” See The Tetragram for more information.