Constantine 1×07: Blessed are the Damned.
The Chi Rho monogram/symbol that appeared during John’s angel summoning spell.
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Update #1 from Catholic.com:
You are speaking of the chi-rho monogram. Though the two letters look like P and X in the English alphabet, they are actually chi (looks like X) and rho (looks like P) from the Greek alphabet. They also happen to be the first two letters of “Christ” in Greek (Christos). Hence the chi-rho monogram is used as a symbol of Christ, Christianity, and Christians.
Update 2 from Wikipedia:
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