Slavery existed both in the Judean culture and in the Roman Empire where Christianity was founded. Christianity knew about slavery from the moment of its inception.

Massachusetts Bay Colony passed the first anti-slavery law in America in 1641. This law made slavery illegal for those who converted to Christianity.

Black Christianity in the North during slavery was characterized by a mix of resistance to oppression and adaptation to the dominant white Christian culture. Many African Americans embraced Christianity as a source of hope and strength, forming their own congregations and blending African spiritual traditions with Christian teachings. Some black churches also served as hubs for organizing anti-slavery activities and promoting social justice.

Answer
John T. Wightman

the simple answer is people! there were originally only indigenous people living in the Caribbean and they had their own religions then when Europeans arrived and introduced slavery they brought Christianity with them then later after the abolition of slavery indentured workers from asia mostly India came, bring with them Hinduism and Islam. The main religions in the Caribbean are Christianity(Catholicism), Hinduism and Islam