Christian symbolism
In the early 3rd century, Clement of Alexandria, used the phrase  (the Lord's sign) to mean the cross. He used the phrase when discussing that the number 318 (in Greek numerals, ΤΙΗ) in  was a foreshadowing  of the cross and Jesus. T is the Greek number fore 300. ΙΗ, the first two letters of Jesus's name ΙΗΣΟΥΣ, means 18. Tertullian called the  Christian believers crucis religiosi, i.e. "devotees of the Cross". In his book De Corona, Tertullian tells how it was a tradition for Christians to trace the sign of the cross on their foreheads.