How did Joseph Priestley’s Unitarian beliefs differ from the doctrine of the Trinity?

Joseph Priestley’s Unitarian beliefs stood in direct opposition to the traditional Christian doctrine of the Trinity, which affirms the unity of the Father, Son, and Holy Spirit as three persons in one Godhead.1

Priestley’s rejection of the Trinity was rooted in his commitment to "Rational Christianity," which prioritized a rational reading of the Bible. His theological position differed from Trinitarian doctrine in the following ways:2

• Rejection of the Trinity: Priestley argued that the doctrine of the Trinity lacked scriptural origin and was a corruption of original Christian teachings.324
• Humanitarian Christology: Rather than viewing Jesus Christ as a divine being coequal with God, Priestley emphasized a humanitarian Christology, asserting that Jesus was a "mere man".32
• Emphasis on Monotheism: His views aligned with the broader Unitarian movement, which sought to maintain a strict interpretation of monotheism—consistent with the Hebrew Scripture's declaration that "the Lord our God is one Lord"—by finding no biblical warrant for the Trinitarian structure.15

Through his writings, such as An History of the Corruptions of Christianity, Priestley maintained that these Unitarian views were more consistent with reason and a literal interpretation of scripture than the established orthodox doctrines.2