Religious imagery:

The halo and aureole/mandorla To stress the divinity of Jesus, a symbolic design element was employed that had no obvious precursor in Greek and Roman art but seems to have been imported from the orient. When we now see a figure represented with a halo behind the head of a painted or sculpted image we instantly identify the figure as holy –or divine. That symbol of holiness or divinity appeared in Christian art in the second half of the 4th century. The halo, however, had its origins in 2nd century India and central Asia in representations of Buddha and other holy persons or beings. At first, in Christian art, the halo was reserved to Christ but eventually was applied to representations of any saint and even, sometimes, the emperor. Christ’s halo became distinguishable from others by the addition of three arms of a cross (or three rays of light) inscribed on the round shape behind his head. Other symbolic shapes, the aureole and mandorla, surrounded the entire figure. They appeared in Christian art at about the same time as the halo. Christ is represented standing or sitting full-figure in front of an oval shape. Often the figure of Christ oversteps the edge of the shape. The mandorla is often alleged by some art historians to be derived from the imago clipeataor Roman shield image but it too is of Buddhist origin.

The halo and mandorla/aureole are visual clues that the image represents orthodox Christian belief.