Richard Brantley states the thesis of his 1984 Locke, Wesley, and the Method of English Romanticism early in his book:
First, Locke’s theory of knowledge grounds the intellectual method of Wesley’s Methodism. And second, Wesley’s Lockean thought (i.e., his reciprocating notions that religious truth is concerned with experiential presuppositions, and that experience itself need not be non-religious) provides a ready means of understanding the ‘religious’ empiricism and the English ‘transcendentalism’ of British Romantic poetry.
As this summary indicates, Locke’s empiricist epistemology translated into religious experimentalism. Following Clifford Hindley, Brantley argues that “Wesley’s sudden conviction of grace was ‘largely determined’ by the bold, but not so sudden, stroke of replacing the ‘metaphysical question’ of knowledge of God with ‘the fundamentally epistemological question ‘How can I know that I am saved?’ . . . ‘Only through experience,’ came the answer.” Thus, “Wesley’s ‘empiricist conditioning,’ if not his training in empiricism, taught him so much respect for experience as the necessary ground of knowledge that Wesley’s quest for ‘a direct experience of the divine love’ was necessarily quasi-philosophic.”
The Lockean influence is unquestionable. In his sermon “On the Discoveries of Faith,” delivered in 1788, Wesley said,
For many ages it has been allowed by sensible men, Nihil est in intellectu quod non fuit prius in sensu: That is, ‘There is nothing in the understanding which was not first perceived by some of the senses.’ . . . [T]his point has now been thoroughly discussed by men of the most eminent sense and learning; and it is agreed by all impartial persons, that although some things are so plain and obvious, that we can very hardly avoid knowing them as soon as we come to the use of our understanding; yet the knowledge even of those is not innate, but derived from some of our senses.
As Brantley says, “These sentences epitomize the main point in Book I of [Locke’s] Essay and indeed in the Essay as a whole.”
Similarly in his discussion, “An Earnest Appeal,” Wesley again summarizes an epistemological position that tracks closely with Locke and extends it into religious epistemology:
[B]efore it is possible for you to form a true judgment of the things of God, it is absolutely necessary that you have a clear apprehension of them, and that your ideas thereof be all fixed, distinct, and determinate. And seeing our ideas are not all innate, but must originally come from our senses, it is certainly necessary that you have senses capable of discerning objects of this kind: Not those only which are called natural senses, which in this respect profit nothing, as being altogether incapable of discerning objects of a spiritual kind; but spiritual senses, exercised to discern spiritual good and evil. It is necessary that you have the hearing ear, and the seeing eye, emphatically so called; that you have a new class of senses opened in your soul, not depending on organs of flesh and blood, to be ‘the evidence of things not seen’ [cf. Heb. 11:1], as your bodily senses are of visible things; to be the avenues to the invisible world, to discern spiritual objects, and to furnish you with ideas of what the outward “eye hath not seen, neither the ear heard.”
Empiricism is translated into religious terms, as a “spiritual empiricism.”
So: Wesley is a Lockean. What about the connection with Romanticism?
Here Brantley makes a case for literary influence. He commends the gracefulness and breadth of Wesley’s prose, and the wide scope of his influences (Calvinist, Arminian, Catholic, Anglican, Dissenting, Lutheran, Puritan). Brantley thinks that calling Wesley’s work “theology” is a diminishment:
[T]heology is neither so inclusive nor so accurate a label for it as philosophical theology. And I suggest that the breadth and substance of the prose together give Wesley a place in les bonnes lettres of eighteenth-century England at least as great as that of ‘Berkeley and Butler and Hume’: Berkeley and Butler blend theology and philosophy, but Wesley is theological where Hume is not.
Having established Wesley’s place in eighteenth-century letters, he argues that Wesley’s thought and style “bear on the manner and especially the thought of William Blake, William Wordsworth, Samuel Taylor Coleridge, Percy Bysshe Shelley, and John Keats.”
Wesley’s “empiricist” and experiential notion of faith contributes, for instance, to Wordsworth’s notion of spiritual experience. More generally:
[T]he primarily natural experience of Locke, and the primarily spiritual experience of Wesley, not only come together in Wesley’s philosophical theology, but thence inform, directly and indirectly, a central dialectic of the poets. Not only does the almost religious quality of their emotion relate to Wesley’s emotional faith, but a Wesleyan blend of ‘spiritual sense’ and a posteriori reason forms part of what they all retained from the century and the place in which all of them were born.
As Brantley summarizes:
[T]he Wesleyan mode of experience defines a criterion of truth in the most important genre of British Romantic literature; the quality of experience in English Romantic poetry is spiritual as well as natural; Wesley’s philosophical theology is a satisfyingly complex, appropriately interdisciplinary, aptly inclusive, and almost only available model for the broad concept of experience everywhere implicit in English Romantic epistemology; the priority of experience in the empirical philosophy, and the importance of experience in the Wesleyan faith, of eighteenth-century England provide, together, a sufficiently synoptic background to British Romanticism; English ‘Romantic origins’ are discoverable on the Wesleyan, i.e., most broadly experiential, ground of British Romantic exemplars.
It’s a striking thesis, and one that challenges the tendency of intellectual historians to hop from philosopher to philosopher, or perhaps from poet to poet, ignoring revivalist theologians they think are beneath intellectual notice.