More From Benjamin Rush on Trinitarian Universalism:

The Trinitarian case for Universalism takes a piece from Arminian logic on the atonement, a piece from Calvinism (indeed a reductio against universal atonement) and puts them together to teach that result. Arminianism teaches Christ made a universal atonement, as opposed to a limited one. Calvinism says Christ died for His Elect only, else His blood would be wasted on the unsaved. Arminianism teaches Christ died for the unsaved too; they just reject His grace. The Trinitarian Universalists seemed to argue that 1) Christ made a universal, not limited atonement. And 2) indeed, His blood WOULD be wasted if even one single soul for whom He died was not saved. Hence, everyone is saved eventually.

I found a source of more letters from Benjamin Rush explaining the case for Trinitarian Universalism, in particular letters of his to Universalist guru, Rev. Elhanan Winchester.

A taste:

Your funeral sermon for Mr. John Wesley does honor to the philanthropy of your universal principles. I admire and honor that great man above any man that has lived since the time of the Apostles: his writings will ere long revive in support of our doctrine---for if Christ died for all, as Mr. Wesley always taught, it will soon appear a necessary consequence that all shall be saved.

-- To ELHANAN WINCHESTER, November 12, 1791.