Why did Joseph Priestley reject Calvinist teachings about original sin and atonement?

Joseph Priestley rejected Calvinist teachings, including original sin and the traditional doctrine of atonement, because he viewed them as "gloomy" and inconsistent with a rational understanding of religion.12

He specifically opposed the Calvinist concepts of the natural depravity of man and the belief in a vengeful God. Instead, Priestley advocated for a "Rational Christianity" that emphasized the perfectibility of humanity. Influenced by the association of ideas developed by David Hartley, Priestley argued that human progress was the inevitable result of increasing awareness within a deterministic system of benevolence.12

Ultimately, he embraced Unitarianism, which he believed was more consistent with a rational reading of the Bible, rejecting the Trinity and maintaining that Jesus Christ was a human preacher rather than a divine figure.12