Staring on page 89 of this collection published in 1767 Mayhew responds to an orthodox critic who writes him out of "Christianity" for denying certain "essentials" of the Christian faith, notably the Trinity. To such "orthodox" Mayhew's "unitarian Christianity" is no better than Deism and perhaps a mask for secret Deism. Mayhew writes that his critic
accuses me of "attempts to undermine the fundamental principles of their faith"—"those essential doctrines"—"the doctrines of grace"—"destroying the fundamental principles of their faith"—and "undermining the dignity and divinity of the Son of God."—All these railing accusations are in page 77. In the next, I am said to "deny and ridicule the doctrine of justification by faith;"—to "discard the notion of original sin"—and to "brand the notion of imputed righteousness with the reproach of nonsense."—And he insults the said reverend gentlemen, as not having "the courage to rise up in defence of the Lord Jesus Christ and the truth of his gospel" in opposition to me.
These were not easy charges to deal with at the time. Mayhew tries to put in context what he really said, that all he really did was deny the Trinity.
The book discusses how high a regard Mayhew really holds for the divine inspiration of the biblical canon. Something about whether the Song of Solomon belongs in the canon and Mayhew's willingness to mess with the orthodox position. As he writes:
But he goes still further; intimating his suspicions that I am a deist, p. 79.—"The Dr.'s reflection upon the Song of Solomon is sufficient to show how easy it is for him to discard the sacred canon of scripture itself: Or perhaps," &c. But he dared not to cite that refleclion, as he calls it. The most that can be fairly and logically inferred from it, is, that I supposed there was near as much reason for admitting the Wisdom as the Song of Solomon into the canon;—a very harmless supposition, even tho' it should be a mistake; and which does not imply the latter to be admitted without reason.
Something very helpful: The orthodox, especially of the evangelical Protestant bent, tend to see orthodox doctrines as clearly taught in scripture. Therefore someone who claims to disbelieve in the Trinity and eternal damnation is elevating his own reason over the Bible regardless of how he might represent his position. Though the Christian-unitarian-universalists did claim biblical support for these positions. As Mayhew writes:
He had before intimated, p. 76, that there was ground to "suspect that I deceive myself, when I profess a regard for—divine revelation"—Behold his candor! He also makes a great outcry, because I somewhere said, that certain passages of Scripture seemed "at first view" to countenance the doctrine of annihilation.
Even though Mayhew was an Arian, not a Socinian, he got flak for referring to Socinus in a positive manner:
There are some more particular insinuations and assertions here, and in other parts of this defamatory pamphlet, in order to shew how widely I differ from our good forefathers. It is intimated that I am a Socinian: "Whatever their notions of liberty—amounted to, they certainly had no great opinion of the 'learned Socinus.'" Here this censor alludes to a passage in one of my sermons on Christian Sobriety, p. 57. in which I speak of Socinus under that character, learned. Will this candid man then allow none to be even learned, unless they are Athanasians?
As has been mentioned before, the orthodox almost put the kibosh on Mayhew's ordination as minister by refusing to attend. Mayhew just it out waited until he could find enough ministers willing to ordain him.