This is a month late, but I thought I would respond anyway.

In simple terms, it was the Maronite Christians, and European powers – prominently France – who are responsible for the survival of Christianity in modern-day Lebanon.

The Maronite Christians came into existence in the 5th century by a monk named St. Maron. The Maronites are Eastern Catholics who "recognized papal supremacy without giving up [their] own Syriac liturgy and priesthood." Their first place of settlement was somewhere near a valley of the Orontes River (the river that runs through the infamous city of Hama, Syria). They traveled from Syria, and ended up isolating themselves in the mountain ranges of Mt. Lebanon, which they later declared their "haven and central homeland."

Outside of isolating themselves in rugged terrain – away from the prevalent Muslim population, and this of course, is an oversimplification, since the Maronites lived among Druze Muslims – the Maronites maintained an alliance with, and gained help from various Western Christians as well. As a result of allying with external forces of great influence – nearing the 1700s, the Maronites eventually became a powerful religious community. Moreover, in the 18th century, the Maronites spread at such a rate, that they dominated their region's demographics, making them a majority in the north.

Following World War I, and subsequently, the collapse of the Ottoman Empire – both Syria and Lebanon fell into the hands of France, which proved useful for the Maronites. The Maronite leaders devised alliances with France and influenced the French to give them dominance in Lebanon. Their governance in turn extended throughout the entire country of Lebanon – even in Sunni and Shia-majority cities and rural lands, meaning not just where the Maronites were concentrated in Mt. Lebanon and other parts of the north. What's also worth noting, is that the earliest Lebanese census report, which was conducted in 1932, estimated that around 51-54% of the population in Lebanon was Christian (but this number clearly and severely dropped in the years coming, thus resulting in tensions over unequal and unfair Christian power and authority in government).

Now this of course leaves out most of the politics, massacres, and civil wars that occurred throughout the centuries (e.g. the Crusades in the 11th century, to the unspoken clashes between Maronite Christians and Druze Muslims in the 19th century). However, simply put, their survival is chalked up to both their isolation – and their alliances with other Christians, countries, and sometimes, powerful families, such as the Khazen family of the Middle East.

Leila Tarazi Fawaz. "An Occasion for War: Civil Conflict in Lebanon and Damascus in 1860." University of California Press, 1995.

Lebanese Information Center. "The Lebanese Demographic Reality." lstatic.org, 2013.