In his book "Real Christianity," Wilberforce also addressed the rise of Unitarianism in Britain [which was at the time much more "Protestant" than is its contemporary manifestation]:
The Unitarian teachers by no means claim to absolve their followers from the unbending strictness of Christian morality. They prescribe love for God that dominates all life and a habitual spirit of devotion. However, people who seek a refuge in this form of faith seem to go there because they want a watered-down sort of faith; they want the joys of Christianity without the difficult doctrines. In particular, most of them seem to want to escape the Bible’s command to be separate from the world, a unique and special people. They prefer to remain at one with the world’s philosophies….
Christianity has not gone to the same efforts to promote its arguments that the Unitarians have. If they were attacked as they have attacked orthodoxy, and asked to defend their tenets, I doubt they would be able to keep their ground so well. In short, we can find no watered-down alternative to Christianity that can be rationally supported. If we have abandoned Christianity, then we must logically abandon all its forms. We must abandon any hope we have of finding the comfort of faith without its demands.
(As quoted in Lon Fendall, William Wilberforce: Abolitionist, Politician, Writer [Barbour, 2002]: 193-95).