“Thalaba,” Mr. Southey’s second poem, is written in open defiance of precedent and poetry. Mr. S. wished to produce something novel, and succeeded to a miracle. “Joan of Arc” was marvelous enough, but “Thalaba” was one of those poems “which,” in the words of Porson, “will be read when Homer and Virgil are forgotten, but — not till then.”

George Gordon, Lord Byron (1788-1824) English poet

“English Bards and Scotch Reviewers,” footnote to l. 205 (1809)
    (Source)

When one of his earlier works was harshly criticized in the Edinburgh Review, Byron wrote this poem satirizing such critics (and the poetry they like). He refers to Robert Southey's "Thalaba," bringing in a phrase used by classical scholar Richard Porson to refer to Southey's poem "Madoc". Except ...

... Porson doesn't include the "but not till then" phrase in his original comment. A man of subtle but biting humor, it seems likely he intended that as a subversive but deniable reading of "when Homer and Virgil are forgotten". Believing that, multiple writers of the time in turn criticized Byron for crudely spelling out Porson's bon mot (examples: Timbs (1862), Powell/Rogers (1903)).