Fish played a role in Jesus’ ministry. His apostles were mostly fishermen, and he called his disciples to be “fishers of men.” He fed thousands of people with a couple of fish, and he ate fish with his disciples after his Resurrection. The fish symbol was widely used in the Roman Empire by pagan religions, so Christians felt compelled to make it their own. Some historians think that Christians may have seen in the Greek work for fish, ichthus, an acrostic for “Jesus Christ, of God, the Son, the Savior” [Iesous (Jesus) CHristos (Christ) THeou (of God) Uiou (the Son) Soter (the Savior)].
Whether or not that is so, the fish symbol was used in the early Church as a hidden reference to their faith in Christ. Precisely because it had a pagan history, the fish symbol aroused little suspicion and could be used safely by persecuted Christians. Etchings of fish are found in the catacombs, where the persecuted Christians buried their dead.