What key concept did believers in Unitarianism reject?

Believers in Unitarianism rejected the concept of predestination, which is the idea that God has already determined who will be saved and who will be damned, regardless of individual actions or beliefs. Unitarianism emphasizes the oneness of God and the individual’s ability to seek personal understanding and connection with the divine. This perspective encourages the belief that humans have free will to choose their paths in life, including the decision to seek salvation.

The rejection of predestination aligns with Unitarianism's broader theological principles, which focus on the inherent goodness of humanity, rational thought, and the belief that individuals are responsible for their own moral choices. By contesting predestination, Unitarians assert that people are not merely puppets of fate, but agents of their own destiny.