From this wonderful book of correspondence of Richard Price see this letter from Rush to Price, dated June 2nd 1787. We have seen Rush advocate for the Trinity, claiming that 99% of late 18th Century America wouldn't appreciate anti-Trinitarianism. I see that as hyperbole. A majority? Maybe. 99%? No. Here Rush likewise may be engaging in hyperbole when he speaks of the popularity of theological universalism, the notion that all men will be saved. Rush seems surprised that the liberal unitarian Richard Price has not as fervently embraced the idea of universal salvation as did Rush (I think Price was a theological universalist, but just not as fervent an advocate of that doctrine as was Rush).
Rush begins by noting that even before he had heard of the Unitarian (i.e., Arian and Socinian) controversies, he had embraced theological universalism:
I confess I have not and cannot admit your opinions, having long before I met with the Arian or Socinian controversies, embraced the doctrines of universal salvation and final restitution.
Rush then strangely notes that his Calvinist beliefs led him to the theological universalist position (elsewhere he claimed to have moved from Calvinism to Arminianism; but Rush might not, like theologians today do, view Arminianism and Calvinism as mutually exclusive positions):
My belief in these doctrines is founded wholly upon the Calvanistical account (and which I believe to be agreeable to the tenor of Scripture) of the person, power, goodness, mercy, and other divine attributes of the Saviour of the World. These principles, my dear friend, have bound me to the whole human race; these are the principles which animate me in all my labors for the interests of my fellow creatures. No particle of benevolence, no wish for the liberty of a slave or the reformation of a criminal will be lost. They must all be finally made effectual, for they all flow from the great author of goodness who implants no principles of action in man in vain. I acknowledge I was surprised to find you express yourself so cautiously and sceptically upon this point. Had you examined your own heart, you would have found in it the strongest proof of the truth of the doctrine. It is this light which shineth in darkness, and which the darkness as yet comprehendeth not, that has rendered you so useful to your country and to the world.
Rush seems to be saying to Price, look, you are an "enlightened Christian" like I am, and it's clear that we kind of Christians disbelieve in eternal damnation. I am shocked that you don't preach as fervently against the doctrine of eternal damnation as you do against the Trinity. Note also how Rush invokes the "tenor" of Scripture. A "spirit" of scripture as opposed to various literal proof texts which might trump. As I have noted before it was this same liberal, "abstraction" approach to scripture that led Rush to oppose the death penalty on biblical grounds.
Then in a letter to Price dated July 29, 1787, Price invokes the Trinitarian Universalist Elhanan Winchester as leading a veritable Universalist revival in America:
The bearer the Rev Mr Winchester has yeilded to an inclination he has long felt of visiting London, and has applied to me for a letter to you, for Americans of every profession and rank expect to find a friend in the friend of human kind. You are no stranger to his principles. I can with great pleasure add, that his life and conversation have fully proved that those principles have not had an unfavourable influence upon the heart. With a few oddities in dress and manner, he has maintained among both friends and enemies the character of an honest man. He leaves many sincere friends behind him. I know not how his peculiar doctrine of Universal Salvation may be received in London. But in every part of America it has advocates. In New England it continues to spread rapidly. In this city a Mr Blair, a Presbyterian minister of great abilities and extensive learning, and equally distinguished for his humility and piety, has openly professed his belief of it from the pulpit.