As N. T. Wright notes, by the time of Justin Martyr, the symbol most closely associated with the Christ-movement was the cross. In the ancient world the cross was a sign of torment and shame. It was a symbol of torment because of the brutal death with which it was associated. It became a symbol of shame because of the shame of the victims of crucifixion, not only because they were despised criminals, but also because they hung naked, with no dignity, after bearing great indignity before dying. That the bodies were then left hanging on the cross until they decomposed as a warning to other criminals only heightened the terror and brutality of this symbol.
That the cross became the chief symbol of the faith and that Paul, premiere NT apostle defined his ministry in terms of the cross as in 1 Corinthians 2:2 says much about the specific nature of Christ’s death. Not only Paul, but Peter also highlights the cross in the first sermon on the Day of Pentecost in Acts 2 and his statement with John before the Sanhedrin in Acts 4.
 N. T. Wright, The New Testament and the People of God (Minneapolis: Fortress Press, 1992), 366-367. The Octavius of Minucius Felix, chapters IX and XXIX, written at the end of the second century are another indicator that the cross was a Christian symbol that early.