Quas Lacrimas links to an old sermon by a unitarian, in which there is a false use of reason, and that without wisdom. The sermon is worth reading, for within there are fundamental errors, but the quote that contrasts the clear teaching of scripture with that theology is:

Having thus given our views of the unity of God, I proceed in the second place to observe, that we believe in the unity of Jesus Christ. We believe that Jesus is one mind, one soul, one being, as truly one as we are, and equally distinct from the one God. We complain of the doctrine of the Trinity, that, not satisfied with making God three beings, it makes; Jesus Christ two beings, and thus introduces infinite confusion into our conceptions of his character. This corruption of Christianity, alike repugnant to common sense and to the general strain of Scripture, is a remarkable proof of the power of a false philosophy in disfiguring the simple truth of Jesus.
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According to this doctrine, Jesus Christ, instead of being one mind, one conscious intelligent principle, whom we can understand, consists of two souls, two minds; the one divine, the other human; the one weak, the other almighty; the one ignorant, the other omniscient. Now we maintain, that this is to make Christ two beings. To denominate him one person, one being, and yet to suppose him made up of two minds, infinitely different from each other, is to abuse and confound language, and to throw darkness over all our conceptions of intelligent natures. According to the common doctrine, each of these two minds in Christ has its own consciousness, its own will, its own perceptions. They have, in fact, no common properties. The divine mind feels none of the wants and sorrows of the human, and the human is infinitely removed from the perfection and happiness of the divine. Can you conceive of two beings in the universe more distinct? We have always thought that one person was constituted and distinguished by one consciousness. The doctrine, that one and the same person should have two consciousness, two wills, two souls, infinitely different from each other, this we think an enormous tax on human credulity.

The unitarian movement is an error, but this error is the beginning of liberal theology. It took the ideas of the enlightenment, challenged what was considered (in 1812) as outmoded superstition, and thus destroyed church, faith and society. You can indeed tell the movement by the consequences of it.

For they are in error. No human created the world. Nor can any human save us, we are all fallen.

Colossians 1:15-23
He is the image of the invisible God, the firstborn of all creation; for in him all things in heaven and on earth were created, things visible and invisible, whether thrones or dominions or rulers or powers– all things have been created through him and for him. He himself is before all things, and in him all things hold together. He is the head of the body, the church; he is the beginning, the firstborn from the dead, so that he might come to have first place in everything. For in him all the fullness of God was pleased to dwell, and through him God was pleased to reconcile to himself all things, whether on earth or in heaven, by making peace through the blood of his cross. And you who were once estranged and hostile in mind, doing evil deeds, he has now reconciled in his fleshly body through death, so as to present you holy and blameless and irreproachable before him– provided that you continue securely established and steadfast in the faith, without shifting from the hope promised by the gospel that you heard, which has been proclaimed to every creature under heaven. I, Paul, became a servant of this gospel.

Our understanding of Christ matters. Ideas matter. Therefore the trinity matters: Christ was fully man but fully God. To Manning this was nonsense, but the progressives then adopted Freud with his structural model (which is nonsense). The Puritans knew better and opposed this liberal nonsense. The errors of the Unitarians became the errors of liberal Christianity and the progressive movement. For if Christ was merely man, we can make our own Jerusalem and there is no need for salvation.

But Christ was sent to us, the incarnate God. He lived as a unfallen man. We could not stand this. We crucified him, and by this act he became our salvation.

We have a saviour who is the same who created this universe. We don’t have a fallen moral teacher.

From this flows another correction: not all will be saved.