https://www.economist.com/img/b/580/852/90/sites/default/fil...
The Christian age bracket in the 21-25 age range is 25%, 50 years ago it was 50%.
Historically, Lebanon has never been Muslim, the mountains served as isolation from the religious changes of the region, and it was a major centre of Christianity during the crusades, and remained connected with Europe.
But in the 20th century, French colonialists enlarged the territory to encompass large areas of Syria, which held Muslim populations:
https://en.wikipedia.org/wiki/Lebanon#Ottoman_Lebanon_and_Fr...
These populations proceeded to overtake the Christian populations, with the fertility rate in 1971 being 5.44 for Muslims and 3.56 for Christians.
This divide remains, in 2004 the fertility rate for Muslims in Lebanon was 1.82, for Christians 1.53:
https://www.lstatic.org/PDF/demographenglish.pdf
A 2009 study found cousin marriage rates of 30% for Muslims and 16.5% for Christians, cousin marriage is commonplace throughout the Muslim world:
https://marciainhorn.com/wp-content/uploads/docs/inhorn-arti...
In 2019 the GDP per capita of Lebanon was $7583. It will eventually (and seems like rapidly) fall to match its Muslim neighbours eg. Jordan ($4405) Egypt ($1667) Syria ($2032).
I realise that these topics are unpopular, but you cannot divorce the economic and social downfall of Lebanon from the demographic changes taking place there.