Squares, circles, triangles; they all have deeper spiritual meanings.
The classical world saw beauty in the order and design of creation. Everything had a meaning, and Christians quickly adopted these fundamental truths when seeking to represent Christian teachings in art.
Circles were seen as a symbol of eternity and were used in halos of saints in heaven as well as a general representation of the Eternal God.
The ancient, secular reason why saints are shown with halos
Triangles were often used to represent the Holy Trinity. When God the Father was visually depicted he would sometimes have a triangular shaped halo.
Squares were seen as a symbol of the earth. Having four sides, the square was also often used in connection with the four evangelists, carrying the four gospels to the four corners of the earth.
Pentagrams, while often used by magicians in the Middle Ages, were first used by Christians to represent the five wounds of Christ.
Oval or almond shapes, called mandorlas, were a frequent symbol in Eastern iconography. A mandorla-shaped full-length halo shown behind Jesus Christ was used to represent his divinity and glory.
One geometric arrangement that was popular in Christian art was called the quincunx. Artist David Clayton explains it to be “the arrangement of five equivalent shapes that has four arranged symmetrically around the fifth which is centrally placed … The five dots on dice, for example, are in a quincunx shape.”
Clayton continues with the Christian symbolism, explaining, “quincunx can be thought of as the geometrical equivalent of the traditional image of Christ in Majesty. Around the central image of the enthroned Christ we see four figures representing the four evangelists carrying the Word to the four corners of the world.”
5 Ancient symbols of the Trinity