France based its 1920 creation of a Lebanese state on the area's large (predominantly Maronite) Christian community, though giving the new entity a viable territorial base necessitated the addition of considerable Muslim-majority areas to the former Ottoman governorate.
The resulting fiction of a clear Christian majority was maintained by counting emigrants (86% Christian at the 1932 census, Lebanon's first and last) among the citizenry while many resident non-Christians without formal documentation were considered non-citizens, turning a (narrowly) Christian-minority "present" population into a more markedly Christian-majority "legal" one.
The subsequent allocation of political power - with all communities represented but Christians given a majority - was embodied in the 1943 "National Pact" in which all main parties agreed to a modified version of the status quo as independence approached: Christians would have a 6:5 parliamentary majority (itself a compromise) and the president would be Maronite, the prime minister Sunni Muslim and parliament's speaker Shia.
That has broadly remained the situation, with the important modification of 50:50 representation in the government and legislature since the ending of the civil war in 1990 (though Muslims are now generally thought to be a majority and Christians at most 40% even before predominantly Muslim refugee arrivals).
Lebanon's remarkable achievement since the conflict of 1975-90 has been to maintain its flawed and fragile poliitcal order, reflecting the desire of all communities to avoid a repetition of past chaos. Christians generally remain opposed to reunification with strongly Muslim-majority Syria, while earlier Muslim enthusiasm for union has given way to widening embrace or acceptance of Lebanese independence, though dissatisfaction with political representation persists.
The state's foundation and related census and citizenship issues are discussed in Melkar el-Khoury & Thibaut Jaulin, Country report: Lebanon, EUDO Citizenship Observatory 2012, and Rania Maktabi ,The Lebanese census of 1932 revisited: who are the Lebanese? British Journal of Middle Eastern Studies 26:2 (1999). The 1989 Taif accords ending the civil war are here,