Though some of its thought can be traced back to the eighteenth century and beyond, theological liberalism, as we know it today, is really a nineteenth century phenomenon. As I understand it, you must have Kant and Coleridge and Schleiermacher to have theological liberalism.
Still, there were plenty of men earlier than the 19C who demonstrated some liberalizing tendencies in denying miracles, the deity of Christ, a vicarious atonement, the foreknowledge of God, justification by faith alone, or the Trinity. Jonathan Edwards was concerned about and responded to many such, as he called them, “Arminians” (orthodox 18C theologians and pastors used the term “Arminian” more broadly than we typically use it today).
In 1752, Edwards’s Scottish friend Thomas Gillespie (1708-1774) had recently been removed from his own pulpit by the Scottish General Assembly after Gillespie had refused to seat a minister the Assembly appointed to the Inverkeithing parish, since that minister’s placement violated the will of that congregation and presbytery. As Edwards shows in a letter to Gillespie, some other characteristics of “liberals” in Edwards’s day are very similar to those of theological liberals since that time:
Perhaps many of the clergy of the Church of Scotland have their minds secretly infected with those lax principles of the new divinity, and have imbibed the generous doctrines (as they are accounted) which are so much in vogue at the present day, and so contrary to the strict, mysterious, spiritual, soul-humbling principles of our forefathers.
I have observed that these modern, fashionable opinions, however called noble and generous, are commonly attended, not only with a haughty contempt, but an inward, malignant bitterness of heart towards all the zealous professors and defenders of the contrary spiritual principles, that do so nearly concern the vitals of religion, and the power of experimental godliness.
This, be sure, has been the case in this land. I have known many gentlemen (especially in the ministry) tainted with these large principles, who, though none seem to be such great advocates for liberty and freedom of thought, and condemn a narrow and persecuting spirit, so much as they; yet in the course of things have made it manifest that they themselves had no small share of a persecuting spirit.
They were indeed against anybody’s restraining their liberties, or pretending to control them in their thinking and professing as they please; and that is what they mean truly, when they plead for liberty.
But they have that inward enmity of spirit towards those others mentioned, that if they see an opportunity to persecute them under some good cloak, and with some fair pretext, will eagerly embrace it, and proceed with great severity and vehemence.*
Theological liberals like to call their ideas free and open and broad-minded, but, in reality, they often demonstrate an ironic antipathy toward conservative theology, despite their claims. What they mean by being broad-minded is that their opinions should be embraced and not shunned. But their theology leaves very little room for opposition or dissenting views, and they often act on this principle. Perhaps this is nowhere better illustrated than in our present day universities (often involving more than mere theological liberalism), which allow for almost no diversity of opinion, when that diversity comes from conservatives or orthodox evangelicals.
*Cited in Yale-Works 16:546. (online)