The meanings, origins and ancient traditions surrounding Christian symbols date back to early times when the majority of ordinary people were not able to read or write and printing was unknown. Many were 'borrowed' or drawn from early pre-Christian traditions. The Definition and Meaning of the Circle as a Catholic Christian Symbol
Catholic Christian symbolism in art provides a clear graphic illustration which represents people or items of religious significance. What is the definition and the meaning of the Circle? The Circle Christian Symbol represents eternity. The circle symbolises eternity as it has no beginning or end. Because of this many early Christians believed that there was something divine in circles. Early Astronomy and astrology was connected to the divine for most medieval scholars, the circular shape of the sun, moon and the planets were related to God's act of Creation. The following image shows the use of a compass to create the perfect circle. And a bright ring, or circle, is used ( halo or aureole) to surround the heads of saints and the whole bodies of other holy persons. 
Circle in Christian Art
Reference to the Circle Christian Symbol in the Bible
The Easton Bible Dictionary provides the following definition, meaning and emblem for the Circle Christian Symbol, represents eternity, in the Bible. The life the faithful have here on earth (John 3:36; 5:24; 6:47, 53-58) is inseparably connected with the eternal life beyond, the endless life of the future, the happy future of the saints in heaven (Matt. 19:16, 29; 25:46). | 