Ichthys (ἰχθύς) Fish used as an acronym for 'Jesus Christ God's Son and Saviour' ... by the early Christians. It is an original symbol of Christianity. It is also an insignia for evangelism... "I will make you fishers of men."
During the times of persecution, the "sign of the fish" was used for a secret Christian identification between believers of the faith.The symbol helped to protect Christian families while they concealed themselves dwelling within the catacombs.
Meanwhile, the Bishops of the church went to their death as martyrs proclaiming the name of Christ, defending the faith, and warning of the coming judgement of God at the end of the age.