The use of the Ichthys symbol by early Christians appears to date from the end of the first century C.E.
Ichthus (ΙΧΘΥΣ, Greek for fish) is an acronym, a word formed from the first letters of several words. It stands for “Jesus Christ God’s Son Savior”–in ancient Greek “Ἰησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ.”