Modern secular thought has its own dualism: It treats only the physical world as knowable and testable, while locking everything else - mind, spirit, morality, meaning - into the realm of private, subjective feelings. The so-called fact/value split.
The most consistent versions of materialism deny the reality of anything beyond matter - no soul, no spirit, no will, no mind. This is called reductionism: Humans are reduced to biochemical machines.
The human mind inherently seeks intelligible order. Thus the conviction that such an order exists to be found is a crucial assumption.