A cross breaking through a circle, carved on a pillar of stone, is the typical Celtic symbol. Pillars of stone were a universal symbol of the link between heaven and earth. The circle round the cross was introduced by the Emperor Constantine as a symbol of the wreath of victory. But only in Celtic lands did it remain fashionable, probably because the circle also came to represent the created world, which the Celts so highly valued. The Celtic cross speaks of victory, wholeness, the transforming power of Christ in all creation.
The old pagan and today’s New Age philosophies offer us a circle without a cross. The Augustinian philosophy offered a cross without a circle. Our world stands in need of both the cross and the circle.
Ray Simpson, Celtic Christianity: Deep Roots for a Modern Faith (Vestal, NY: Anamchara Books, 2014), p.157.