Over the millenia Christians have used many symbols, the Cross being perhaps the most well known. Protestants commonly use a plain cross, while a Crucifix is more common among Catholics. A symbol very common among modern day Christians, is the fish.
How did the fish, which is never mentioned in scripture as a symbol related to Christ come to be a Christian symbol?
In Greek, the language of the early Church, ichthus is the word for fish. In Greek ichthus is spelled ΙΧΘΥΣ. That is Iota(Ι), Chi(Χ), Theta(Θ), Upsilon(Υ), Sigma(Σ). Each of these letters can be made to stand for a word, a kind of code for those who know.
Iota(Ι) is the first letter of Iesous, Jesus in Greek. Chi(Χ) is the first letter of Christos, Christ, which means "anointed One" in Greek and can be translated more correctly into English as Messiah. Theta(Θ) is the first letter of the Greek word Theos. While god is most commonly described using the word Dios, this word is typically applied to the pagan Greek gods. The translators of the Septuagint used the word Theos to refer to the God of Jacob. Upsilon(Υ) is the first letter of the Greek word uios, meaning son. Sigma(Σ) is the first letter of the Greek word for Soter, which means savior.
So Ichthus (ΙΧΘΥΣ) can stand for Jesus Christ, Son of God and Savior.
The symbol of the fish was use by followers of the Way to mark places where they met and is also believed to have been used as a sign between Christians who did not know one another.
Today many Christians display the ichthus proudly to proclaim they are followers of the Son of God.