In the earliest of Christian times, when Christians were heavily persecuted, the fish symbol came to represent Christianity long before the Cross symbol was in use. The symbol of the fish was used to identify oneself as a Christian to others, mark burial places of the saints and martyrs and would also indicate the location of where a prayer meeting was held.
There are many different interpretations as to why or how the fish symbol originated, some of which can be traced back to a time long before Jesus Christ came into the world – as far back as the third and fourth century BC with the stories of the mythical Orpheus of Thrace that was described as a fisher of men. Many other ancient cultures and religions used the symbol of the fish as well to denote various observances. But, in the case of Christianity, the fish symbol was most likely adopted from the writings of the Apostles which speak of the miracles that Jesus preformed in feeding the multitudes and the calling of the first Apostles, Simon Peter, James and John (Luke 5:1-11, Matthew 4:18-22 and Mark 1:16-20) who were referred to as the “reputed pillars” of the Jewish Christian community in the Apostle Paul’s letter to the Galatians (Gal. 2:1-10). Most Orthodox Christian theologians also think that the great catch of fish in Luke 5:1-11, Matthew 4:18-22 and Mark 1:16-20, fulfills the prophesy in the Old Testament of Jeremiah 16:16, “…says the Lord….behold ….I will send many fishermen, and they will fish them.”.
The Christian Fish symbol is also aligned with Jesus’ Resurrection and the story of Jonah and the Whale of the Old Testament (Jonah 1:17- 2:10) through the Apostle Paul’s letter to the Corinthians 15:1-58; and, the Gospels of Matthew (12:38-40) and Luke (11:29-30) (KJV).
IΧΘΥΣ is an acronym for Ίησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ – phonetically pronounced: Isous Christos, Theou Yios, Soter – in English: Jesus Christ, Son of God, Savior. Some believe the acronym inside the fish started to become popular in the 1970s. But I believe the existence of the acronym inside the fish was in use long before that time; and, although I cannot find a reference now, I remember as a child seeing the Fish symbol with the IΧΘΥΣ written inside in documents that dated well before the 1970s, perhaps even as early as the 1950s or before. At some point, modern culture converted the Greek acronym IΧΘΥΣ written inside the first symbol to simply the English word Jesus and other variations also exist in contemporary society today.
References: Lawrence Farley, The Gospel of Luke, Good News for the Poor, Conciliar Press; Walter Liefeld, The Expositor’s Bible Commentary Vol. 8; Zondervan; Paul Tarazi, Luke and Acts, St. Vladimir’s Seminary Press, KJV.