Prior to colonization by the American Colonization Society (ACS), which began the majority of people in what we now call Liberia practiced traditional animist beliefs, similar to those found throughout West Africa. However, a fair number of Liberians also practiced Islam as the collapse of the Sonhai Empire to the north resulted in thousands of Muslims migrating closer to the coast.

The Christian influence came from the arrival of free blacks and whites who began moving to Liberia in the early 19th century and continued to emigrate for decades before the movement petered out. However, as with most of West Africa, very few societies had any type of written language so any pre-colonial religious practices we know of are typically taken from the accounts of colonists and missionaries and the like.

The ACS itself had tremendous support from many religious groups in the US who saw the colonization of Liberia as the best route for free blacks as the ACS was founded on the premise that free blacks and whites could not peacefully co-exist. It is important to note that many of these same people, were also ardent abolitionists.

So while the colony of Liberia itself is rather unique when compared to European colonies, the proselytizing of indigenous people was pretty standard fair. Like most of West Africa, Christianity took root in Liberia and is now the dominant religion although, especially in rural areas, there is still quite a bit of syncretism with traditional practices and beliefs.

A lot of my info is from Marie Tyler-McGraw's An African Republic, Black and White Virginians in the Making of Liberia.