Complex sentence: He disputed the idea that missionary work should be focused on the non-Islamicised eastern portion of the colony should be prioritsed over the western portion, where Islam had already become entrenched.
Split into simpler sentences: He disputed the idea that missionary work should be focused on the non-Islamicised eastern portion of the colony. He believed instead that missionary work should be prioritsed in the western portion, where Islam had already become entrenched.