What is the purpose of the Chi-rho?

Question

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Answer 1

It was an easy way of depicting the word "Christ" for a group of believers that were almost entirely illiterate, but were also largely familiar with the Greek alphabet.

Later, when the familiarity with Greek was lost as well, the most common symbol for Christ became what most people know today - the cross.

Answer 2

The Chi-Rho was the symbol used by the first Christians and then later adopted by Constantine and used for many years as the symbol of the Christian Roman Army.

Answer 3

First two letters of Christ in Greek.