<P> However , the cross symbol was already associated with Christians in the 2nd century , as is indicated in the anti-Christian arguments cited in the Octavius of Minucius Felix , chapters IX and XXIX , written at the end of that century or the beginning of the next , and by the fact that by the early 3rd century the cross had become so closely associated with Christ that Clement of Alexandria , who died between 211 and 216 , could without fear of ambiguity use the phrase τὸ κυριακὸν σημεῖον ( the Lord 's sign ) to mean the cross , when he repeated the idea , current as early as the apocryphal Epistle of Barnabas , that the number 318 ( in Greek numerals , ΤΙΗ ) in Genesis 14 : 14 was interpreted as a foreshadowing ( a `` type '' ) of the cross ( T , an upright with crossbar , standing for 300 ) and of Jesus ( ΙΗ , the first two letter of his name ΙΗΣΟΥΣ , standing for 18 ) , and his contemporary Tertullian could designate the body of Christian believers as crucis religiosi , i.e. `` devotees of the Cross '' . In his book De Corona , written in 204 , Tertullian tells how it was already a tradition for Christians to trace repeatedly on their foreheads the sign of the cross . The crucifix , a cross upon which an image of Christ is present , is not known to have been used until the 6th century AD . </P>
when did the crucifix become the symbol of christianity
in the 2nd century