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.