In the Greek and Roman Church the thumb and first two fingers represent the Trinity. The thumb, being strong, represents the Father; the long or second finger, Jesus Christ; and the first finger, the Holy Ghost, which proceedeth from the Father and the Son. (See Blessing.)
Some bishops of the Anglican Church use this gesture while pronouncing the benediction.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894