art

Both Western and Eastern traditions have used this particular finger arrangement for centuries, although in the West (Roman Catholics), this particular gesture have been put aside in the last few decades. this placement of the fingers is not uniquely reserved for priests either.

Dom Prosper Gueranger, OSB (1805-1875) in his monumental work The Liturgical Year (volume 15) explains the signification of this gesture on his entry for the Feast of St. Cecilia (November 22). Abbot Gueranger explained that the body of this martyr was discovered in the Catacombs of St. Callistus in 1599 and that her right hand was found in this very position. while her left hand had index finger pointing straight out. The two fingers of the left hand signified the two natures in Christ (human and divine) and the other three fingers together signified the Holy Trinity. As for the index finger of the left hand. Dom Gueranger tells us that it signified the truth of one God. "St. Cecilia," 1599 (Wikipedia).

It seems that the open hand of Christ symbolizes peace and the five wounds of Christ. Other icons have the finger placement in a slightly different position on Christ's right hand and have a somewhat different meaning. Both Greek and Latin variations and their meaning can be found here: What is the significance of Christ's hand gesture (thumb and two fingers up, two fingers down) in Christian art?.

The sign of the cross was made at liturgical functions over persons and things, sometimes with five fingers extended, to represent the Five Wounds of Christ, sometimes with three, in sign of the Persons of the Trinity, and sometimes with only one, symbolical of the unity of God. For the blessing of the chalice and oblations Leo IV prescribed that two fingers be extended and thumb placed beneath them. This is the only true sign of the Trinitarian Cross. The pope warmly recommended his clergy to make this sign with care, else their blessing would be fruitless. The action was accompanied by the solemn formula, "In nomine Patris, etc." Another use of the cross was in the solemn dedication of churches. - Archæology of the Cross and Crucifix