https://www.hashtags.org/definition/Xp/

1 Definition for Hashtag #Xp

An abbreviation of Christ using the Greek letters Xi and po (p makes an r sound in Greek, so pronounced "Key" and "Row"), which are the first two letters in the word Christ in Greek. This is a traditional sign of Christ. It can be used here to denote anything relating to Jesus Christ, Christology, or Christianity(see #Xn).