The Greek word for “fish,” loosely, ichthus, is the same as the initial letters of the phrase “Jesus Christ, Son of God, Savior”:
lesous CHristos, THeou Uios, Soter.
Since Jesus was already associated with fishermen and fish, it worked well.
Later, when persecution became widespread, Christians were able to greet one another with a drawing of a fish to keep from giving themselves away to authorities.
It became the universal symbol for Christianity.