As Miller writes [with paragraph breaks added for clarity]:
One more insurmountable Objection to, the Unitarian system with me, is, that Infidels EVERY WHERE PREFER THIS SYSTEM TO ANY OTHER THAT BEARS THE CHRISTIAN NAME, and feel no reluctance to uniting in worship with its adherents.
It is not an uncommon thing for Unitarians to boast, that avowed Deists, on hearing, or reading the discourses of their distinguished preachers, have greatly admired them; and declared, that if the system exhibited in them were Christianity, they had no longer any difficulty in taking the name of Christian.
I have been credibly informed of repeated instances of this kind in reference to the Rev. Mr. Channing's sermon, preached and published in Baltimore. Unitarians consider this fact as a most potent argument in favour of their creed; as an argument, that it is so rational, and so strongly commends itself to common sense, that even infidels bow to its authority. But is it not a much more direct and powerful proof of something very different; viz. that Unitarianism and Infidelity are so closely allied, that he who embraces the one, has really no good reason for objecting to the other? This, I have no doubt, is the real ground of the fact in question. And, indeed, how can it be otherwise?
The prevalent system of Unitarianism at the present day, not only makes Christ a mere man, and discards the whole doctrine of Redemption; but also, as you have seen, rejects the inspiration of the scriptures; and, in short, presents a system reduced so nearly to a level with the Deistical scheme, and allows so much latitude of belief and of feeling, with regard to what is left, that the Deist must be fastidious indeed, who would feel much repugnance to joining in communion with a Unitarian society.
Dr. Priestley seems to have been very much of this opinion; for, in writing to a Unitarian friend, concerning a gentleman who had been commonly reputed a Deist, he observes— "He is generally considered as an unbeliever: IF SO, HOWEVER, HE CANNOT BE FAR FROM us; and I hope in the way to be not only almost but altogether what we are."*
Mr. Belsham, according to a representation given in a former Letter, explicitly acknowledges, that Unitarianism does not differ, in any important point, from serious Deism; and; in another place, does not hesitate to avow, that he would much rather embrace Deism than Orthodoxy.*
So Infidels themselves view the matter. They have little objection to the prevalent forms of Unitarianism; not because they are willing to approximate to real christianity; but because they see something, under the name of christianity, NEARLY APPROACHING TO THEM.