The geography of Lebanon has played a significant role in the shaping of its religious diversity. Hills, valleys, mountain passes and other features, combined with further artificial reinforcement, allowed communities to thrive that otherwise would have been destroyed by larger imperial powers.

As a result, virtually every ruling power in Lebanon has found some portion of its population to be restive from the time of the initial Arab conquests (when Christian pseudo-bandit warlords were able to resist the initial conquests and maintain significant autonomy for centuries) up until this century with the Israeli and Syrian occupations.

As for how it's held itself together, well, it has and it hasn't. The National Pact established strict guidelines for sectarian political representation. This idea that power must be distributed along sectarian lines according to demography extends through theoretically non-political entities. When I say it hasn't however, obviously the Lebanese Civil War very nearly ripped the country apart. The Taif Agreement adjusted the National Pact to reflect the changing demographic balance of power over the course of the 20th century that saw the Maronite population experience a relative decline in numbers (but not wealth, status, and power) in comparison particularly with Lebanon's Shia.

That said, however, and without getting into 20 year territory, the Taif agreement's allowance of Hezbollah to exclusively retain its armed militia has allowed to exercise a level of governance in the south and in the Beqaa valley that is comparable to or surpasses the authority of the central government.

Sources. For Lebanon in the Arab Conquests: I've recently been reading Robert Hoyland's In God's Path which has some interesting details on this. For the past 500 years or so see Eugene Rogan's The Arabs: A History

Edit: tried to clarify some ambiguous phrasing.

It's the most genetically anglicanized city in the middle-east and also one of the most diversified/successful economies in the region as well. The melting pot history Lebanon has had contributes to its culture and also its success.

Dumper, Michael; Stanley, Bruce E.; Abu-Lughod, Janet L. (2006). Cities of the Middle East and North Africa.

Lebanese political science and history student here.

Well the Levant and Lebanon in particular witnessed well over 15 foreign occupations. Some lasting for months, others lasting 500 years (Ottomans). Basically Lebanon is such a melting pot due to the its geographical location between three continents in addition to the middle-age and modern religious campaigns that took place in the area such as the Muslim conquests from the Arab peninsula, the crusades, the Mamlouks and the Ottomans. The country has witnessed many episodes of sectarian conflict and so in a sense it is a failed state, yet a state nevertheless. Lebanon is built on the no winner/no loser sectarian approach institutionalized through the confessional system. The sects in Lebanon are also geographically "isolated" outside of Beirut the capital. Even Beirut's neighborhoods are mostly sect-specific. Historically, Lebanon used to comprise of the self-autonomous Mount Lebanon, and later on Beirut became a province or Mutasarifiya as well. When the state of Lebanon was created in 1920 by the French, they added other areas from Greater Syria. These areas mostly inhabited by Shiites added to the already existing sects in the mountain such as the Druze, the Maronites, the Sunnis of Beirut, and other smaller Christian and Muslim sects.

It is also important to note that there are so many different Christian sects due to the different Western Church missionaries that started arriving to the Levant after Napoleon's conquest in Egypt in the late 19th century. Other sects' presence can be attributed to Lebanon's mountainous terrain which offered them safe haven during the times of prosecution such as the Druze after the Fatimid Empire, the Maronites after the council of Nicea, the two Armenian sects after the 1915 genocides, and several others.

Source: The Modern History of Lebanon- Kamal Salibi

Also my thesis research work.

Edit: grammar and wording