This gesture, in which his first two fingers and his thumb are extended and his third and fourth finger are closed, is among the most frequently occurring of Christ's hand gestures in Christian art. It emerged as a sign of benediction (or blessing) in early Christian and Byzantine art, and its use continued through the Medieval period, and into the Renaissance.
The sign is most frequently seen in iconographic images of Christ, which appeared in churches in the Early Byzantine and Medieval periods in the form of mosaics, stained glass windows, relief sculptures, and paintings. One of the most common of such images is the Christ Pantocrator (or Christ Almighty), which depicts Christ, usually isolated against a golden background, with his head encircled by a halo, his left arm hugging the gospels to his chest and his right hand raised in the sign of benediction. It is important to note that this gesture is always made with the right hand, as this is the hand with which one blesses, according to Christian doctrine.
The sign was originally derived from a symbol used in Roman art to indicate speaking, and first gained popularity as a Christian symbol shortly after Constantine's issue of the Edict of Milan in 313 AD, allowing Christians to practice their religion freely, without the threat of persecution. Indeed, Constantine himself converted to Christianity, and Christian art flourished. In early images of Christ, one can see an early manifestation of the sign of benediction in which the thumb is closed over the palm, rather than open. As Christian art evolved, symbols, including Christ's hand gestures, took on deeper significance. With the thumb opened, the three open digits came to represent the Trinity (The Father, Son, and Holy Spirit), while the two closed represented the dual nature of Christ as both man and God.
When the Church split early in the second century AD, the symbol for blessing used by the two newly-formed churches likewise split. The Greek Orthodox church began to make use of a symbol derived from a common abbreviation of the Greek version of Christ's name. In this manifestation of the sign of blessing, the first finger is held erect, representing an 'I'; the second is bent in the shape of a 'C'; the thumb and third finger cross to form an 'X'; and the pinky, like the second finger, curves into a 'C'. Thus, the five digits together spell out "IC XC" an abbreviation of the Greek name of Jesus Christ, taken from the first and last letters of both parts of his name. The Roman Catholic Church, meanwhile, maintained the use of the three open digits and two closed (now so familiar to Westerners). The early sign, in which the thumb is closed, had by this time faded almost completely.
With the onset of the Renaissance in Western Europe, the sign of benediction became less common in Christian art, due primarily to the shift away from iconographic images of Christ-as-Savior towards more naturalistic depictions, emphasizing the human in Christ over the God. Unlike the highly stylized, reverence-inspiring images of old, Renaissance art presented a more realistic image of Christ as he would have appeared to those who saw him in the flesh. He was more likely to be painted in the humiliation of betrayal, or the agony of death, than in the glory of Christ Pantocrator.
Still, the gesture does occasionally appear; though void of the characteristic rigidity of earlier icons, it takes on a rather casual feel, more akin to a guy flashing a "peace" sign than to The Savior bestowing his blessing.