Final answer:
The two secret symbols used for Christians were the fish symbol and the Chi-Rho symbol. The fish symbol, called the Ichthys, represented the phrase 'Jesus Christ, Son of God, Savior' and was used as a secret identifier by early Christians. The Chi-Rho symbol, made of the Greek letters chi and rho, was used by Emperor Constantine and became a widely recognized symbol of Christianity.
Step-by-step explanation:
Two secret symbols used for Christians were the symbol of the fish and the Chi-Rho symbol. The fish symbol, known as the Ichthys, was used by early Christians as a secret symbol to identify themselves to each other. It was an acronym that symbolized the phrase 'Jesus Christ, Son of God, Savior.' The Chi-Rho symbol, comprised of the Greek letters chi (X) and rho (P), was used by Emperor Constantine and became a prominent symbol of Christianity after his conversion. It represented the first two letters of the Greek word for Christ.