Gosh .. Leaving aside a history of human occupation going back to 5,000 BC or so and looking just at more recent history, Lebanon came under Ottoman rule in the early 16th century and that "proto state" became a concentration area for Lebanese Maronite Christians who (IIRC) made up only 25% or so of the population.

I think it would be better described as a nation with Christians (and other faiths) rather than as a purely Christian nation.

There's also the post WWI Mandate for Syria and the Lebanon, administered by France, followed by the post WWII new Lebanese state that gained gained independence from Free France - that was a balancing act between multiple religious groups and also hardly a purely Christian nation.

As to the question asked, Hezbollah was founded in 1982 directly as a result of the Israeli invasion of Lebanon.

That's a very light on detail overview - it's just cans of worms all the way down.