It refers to “before present”; in radiocarbon dating ‘present’ is 1950, the year used as the baseline because that’s when RC dating was pioneered.
If BP were anti-Christian, it would stand for “Before Pilate” or maybe “Before Paul”. But A) it isn’t anti-Christian, B) SunkenCiv isn’t anti-Christian, and C) nothing about this topic is anti-Christian.
Thanks for your explanation. I was familiar with the anti-Christian BCE, but I wasn’t familiar with BP, except in the context of large oil companies.