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Early Christian believers often used the "fish" symbol. This may be because Jesus called his disciples to be "fishers of men" (Matt. 4:19). Or, perhaps the Greek word for fish, ixthus, was an acrostic for Jesus Christ, Son of God, Savior:
i - first letter in the Greek word for "Jesus"x - first letter in the Greek word for "Christ"th - first letter in the Greek word for "God"u - first letter in the Greek word for "son"s - first letter in the Greek word for "savior"