Halos are visual shorthand, part of the symbolic vocabulary of Christianity that was for centuries the only catechism for multitudes of believers who couldn't read. In a more literate age, such symbols are no longer necessary. But we still use them, since they reveal at a glance that this person is a guide and helper on our own road to sanctity.
The idea that light emanates from holy ones is also biblically attested. God's glory is understood as a form of radiance. When Moses goes up Mt. Sinai to encounter God face to face, he returns so radiant that he must veil his face from the community so as not to risk contact between the sacred and profane—always a hazardous business. Thereafter, whenever Moses enters the Tent of Presence to meet with God, he covers his face afterwards. Close encounters with God appear to place us in contagious proximity to divine glory. The emanation from Moses was later translated from Hebrew by biblical scholar Saint Jerome as horns rather than rays of light, which is why some artists depicted Moses with horns.
It may not have been a mistranslation. Egyptian and Mesopotamian gods and heroes wore horns as a sign of their glory, honor, and authority. Later on, horns and rays are rounded out into the more familiar halo, often painted with gold foil or set with precious metals and jewels in icons. Circles are perfect, like divinity. Christ receives the first round halo in art, then the angels, and finally the saints. Interesting, rare portraits of God the Father employed a triangular halo instead to recall the Trinity. Baby Jesus sometimes has one too–perhaps because he so recently departed the Trinitarian realm for earth. Jesus may also be crowned with a cruciform halo, which is uniquely his.
Faith, Hope, and Love are sometimes shown in art as human figures and when they are, they wear hexagonal halos. So too the cardinal virtues Justice, Prudence, Fortitude, and Temperance. The very rare square halo was used to denote a living person popularly proclaimed a saint, but technically not yet eligible for the crown of light. As minimalism became fashionable in art, the halo was reduced to a disc hovering overhead, or even a mere circlet of gold. Animals that symbolize holy ones—the Lamb of God, the Holy Spirit dove, and the four Evangelists of Revelation—might also wear halos. You and I, too, hope to do so.
Scripture: Exodus 34:27-35; Deuteronomy 5:23-27; 2 Chronicles 5:14; 26:18; Psalm 19:2; 79:9; 89:16; Isaiah 35:2; 60:1-3; Baruch 5:1-3, 9; Ezekiel 8:4; 43:2; Daniel 12:3; Wisdom 7:10, 25-30; Sirach 43:9; Mark 15:38; Matthew 27:51; Luke 23:45; John 1:3-9; Acts 7:55-56; Romans 8:16-17; 2 Corinthians 3:12-18; 4:3-6; Revelation 21:11, 22-24
Books: The Square Halo, and Other Mysteries of Western Art: Images and the Stories That Inspired Them - Sally Fisher (New York: Harry N. Abrams, Inc, 1995)
Dictionary of Christian Art - Diane Apostolos-Cappadona (New York: Continuum Publishing Company, 1994)