Ichthus (ΙΧΘΥΣ) is the Greek word for fish. The fish was often used as a symbol of Christianity by early Christians. In fact, it may be one of the oldest Christian symbols. It makes sense. Fishing is a common theme in the New Testament; several of Jesus' disciples were fishermen, Jesus fed large groups with bread and fish, Jesus cooked fish, Jesus says in Matthew 13:47 that the "the kingdom of heaven is like a net that was thrown into the sea and gathered fish of every kind." The Greek word ichthus can be used as an acronym for Jesus, Christ, Of God, Son, Savior, or in English order, Jesus Christ the Son of God, Savior. (See image below).
When Jesus called Peter and Andrew as disciples, He said to them, "Follow me, and I will make you become fishers of men.” They would no longer be catching fish but would be catching people to be a part of God's kingdom. So, in a way each of us is like a little fish! We have been caught in the net of Christ's church by the various fishermen that have proclaimed to us God's word and administered to us His sacraments.
An early church theologian, Tertullian (c. 160-220), picks up on this fish-person analogy in his treatise on baptism. He writes that as water sustains fish, “we, little fishes, after the image of our Ichthus, Jesus Christ, are born in the water (of baptism) nor are we safe but by remaining in it.” Fish and water go together, so also the Christian and baptism. Think about your baptism from time to time, and when you do so, remember that through baptism you are one of God's fish graciously caught up in the net of His kingdom.