The Purple Arches - The entrances to caves or catacombs, where early Christians met in secret: also, the shape of Gothic church windows.
The Fish - An ancient symbol for the Christian faith, perhaps a secret code mark; Letters of the Greek word for fish can be used as first letters in the phrase, "Jesus Christ God's Son Savior."
The Chalice - The Lord's Supper, and thus the earnest and simple fellowship of the early church.
The Upside-Down Cross - Peter, chief of the apostles, who in legend, is said to have been crucified upside down because he thought himself unworthy of a death like his Master's.