Could someone help identify this 18th century oil painting?

I was told that it came from a french monastery and was from the 18th century, but don't know much else about it, would love some help
The dimensions are 6.5ft(w) by 7ft(h).

I'm looking to sell this for a friend who believes it's worth 20k, So if that isn't the case, I would like a decent explanation as to why so that I can explain it to him. I would also like to know as much as possible about this painting since it's been a mystery so far.

Thanks in advance!