In Christian symbolism, a square represents concepts related to the number four, such as the four corners of the earth and the Four Evangelists. (For more on the number four in Christianity see the symbols of numbers, #5 right).
In Christian art, the square is most frequently found as a nimbus (halo), where it denotes a saintly person who was still living at the time of the artwork.
This is based on the identification of the number four with earth or live on earth.
Related Bible Verses
"And the four living creatures said, 'Amen!' and the elders fell down and worshiped."
~ Revelation 5:14
"After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth, that no wind might blow on earth or sea or against any tree."
~ Revelation 7:1
"...and saw the heavens opened and something like a great sheet descending, being let down by its four corners upon the earth."
~ Acts 10:11
"And the angel answered and said to me, “These are going out to the four winds of heaven, after presenting themselves before the Lord of all the earth."
~ Zechariah 6:5
- - "halo." Peter and Linda Murray, Oxford Dictionary of Christian Art (2004).
- Symbols in Christian Art and Architecture by Walter E. Gast. Top illustration of the square also by Mr. Gast.