I don't know. “These doctrines I could not receive. I could not receive [the Calvinist] views on the subject of atonement, regeneration, faith, repentance, the slavery of the will, or any of the kindred doctrines.” Iain Murray, Revival and Revivalism, 256. Citing from Finney's Memoirs.
I find it disturbing that he held to the governmental view of the atonement and believed that forgiveness was merited by human contrition. He also believed, or at least seemed to indicate at times, that conversion was the sinner choosing to obey God. That doesn't sound like a great definition of faith. It's been a while since I read his Systematic Theology, but I believe he also denied original sin, articulating the human condition in terms of Pelagianism. Real Pelagianism, not the watered-down Wesleyan version. If that is true, then I would be inclined to place in the camp of true heretics.
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As to why some good was done by him, Iain Murray points out that he started out a fairly normal Presbyterian minister and gradually grew to the extremes he is known for. Even his detractors believed he was doing some good work at the beginning of his ministry. However, it is worth noting that Finney's popularity coincided with the end of the Second Great Awakening, exposing the falseness of his claim that the new measures would produce endless revival.