The good Rev. and "key" Patriot Preacher was not only a "unitarian" but also, it seems a "universalist."
From his 1762 Thanksgiving Sermon:
"The consideration of God's goodness and mercy, particularly as manifested in the Scriptures, in the redemption of the world by Christ, naturally suggests very pleasing hopes, and a glorious prospect, with reference to the conclusion, or final result of that most wonderful interposition of grace. It cannot be denied, that ever since the apostacy of our first parents, there have been, and still are, some things of a dark and gloomy appearance, when considered by themselves. So much folly, superstition and wickedness there is, 'in this present evil world.' But when we consider the declared end of Christ's manifestation in the flesh, to give his life a ransom for all, and to destroy the works of the devil; when we consider the numerous prophecies respecting the destruction of sin and death, and the future glory of Christ's kingdom on earth; when we consider, that he must reign till he hath put all enemies under his feet, the last of which is death; and until he hath subdued all things to himself; when we reflect, that according to the apostle Paul, where sin has abounded, grace does much more abound; and that the same creature (or creation,) which was originally made subject to vanity, is to be delivered from the bondage of corruption, into the glorious liberty of the children of God; when we consider the parallel which is instituted and carried on by the same apostle, betwixt the first and second Adam, in his epistle to the Romans; and his express assertion in another, that "as in Adam all die, even so in Christ shall all be made alive; but every man in his own order;" in a word, when we duly consider that there is a certain restitution of all things, spoken by the mouth of all the holy prophets since the world began; when we duly consider these things, I say, light and comfort rise out of darkness and sorrow.
And we may, without the least presumption, conclude in general, that, in the revolution of ages, something far more grand, important and glorious, than any thing which is vulgarly imagined, shall actually be the result of Christ's coming down from heaven to die on a cross, of his resurrection from the dead, and of his being crowned with glory and honor, as Lord both of the dead and the living. The word of God, and his mercy, endure forever ; nor will he leave any thing which is truly his work, unfinished. 'As the heavens are higher than the earth,' saith the Lord, 'so are my ways higher than your ways; and my thoughts than your thoughts— My word, that goeth forth out of my mouth, shall not return unto me void, but it shall accomplish that which I please; and it shall prosper in the thing whereunto I send it.'"
"To conclude then ; let us all, young men and maidens, old men and children, love and honor, extol and obey the God and Father of all, whose tender mercies are over all his works; and who has been so gracious and bountiful to ourselves in particular. If we sincerely do thus, as becometh the children of the Highest, we shall, in due time, partake of his goodness, in a far more glorious manner and measure than we can in the earthly house of this tabernacle. We shall doubtless also have a far more clear, distinct and perfect knowledge, than we can possibly have at present, of what is intended in some apparently grand and sublime, yet difficult passages in the sacred oracles; particularly that of John the Divine, with which I close: 'And every creature which is in heaven, and on the earth, and under the earth, and such as are in the sea, and all that are in them, heard I saying, blessing, and honor, and glory, and power be unto Him that sitteth on the throne, and unto the Lamb forever and ever." Rev. v. 13."
Now this sounds "Christian." But Rev. Mayhew 1. denied the Trinity; 2. as we just saw, denied eternal damnation; and 3. believed men were saved by their works, following Christ's perfect moral example. (The bad -- those that didn't earn immediate salvation at death -- would be punished temporarily but eventually reconciled to Christ). Whether it's "Christian" or not is above my "pay-grade" as a co-blogger would put it. But it DOES help to know what the Founding era unitarian-universalists believed.