[I'm not a teacher.]
Here's an difinition of christen from Cambridge Dictionary.
Thus, christen refers to give a baby name which is relevant to their parents' religious belief- Christanity and the ceremony. In other words, the baby becomes a Christian, and the name is a Christian name.christen
1 to give a baby a name at a Christian ceremony and make him or her a member of the Christian Church:
She's being christened in June.
[+ noun] She was christened Maria.
For example, my parents aren't Christians and they just gave me an ordinary name without any regilious ceremony.
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