The Greek word ichthys (Greek word IXOYE
meaning "fish") is an acrostic for "Jesus Christ, the Son
of God, the Savior." The fish symbol represents the acrostic,
and symbolizes salvation in Christ through water baptism
-- fish are saved in water, and we are saved in water through
Jesus (1 Pet. 3:21).
Tertullian (145-220 A.D.) wrote a treatise on baptism contradicting
the Cainites (a Gnostic sect) who taught against water baptism.
Speaking of Christians he wrote, "we little fishes are born
in water, after the example of our Ichthys Jesus
Christ. And we have safety in no other way than by permanently
abiding in water."
Later in the treatise, Tertullian says that the Cainite
doctrine against baptism is a "false doctrine" that "shakes
the faith" which can "entirely block a person from receiving
the faith. In fact, it opposes the faith on the very principles
of which the faith consists!"
Then describing the simplicity of baptism Tertullian says,
"With the utterance of a few words, he is dipped, and then
rises again not much the [physically] cleaner."
The ichthys (fish symbol ) contradicted the Gnostic
doctrines brethren fought against in the early centuries
(that men could be saved without baptism). It symbolizes
the necessity of baptism for the remission of sins (Acts
2:38), that our sins are washed away in baptism (Acts 22:16),
and that baptism saves us (1 Pet. 3:21).