God hath made no decree to distinguish the seasons of his mercies; In paradise, the fruits were ripe, the first minute, and in heaven it is alwaies Autumne, his mercies are ever in their maturity.
From context he doesn't seem to be implying a pun on fall; I found the conceit rather striking, but if one imagines autumn as the season of "mellow fruitfulness," or as Horace's "pomifer autumnus," there is nothing implausible about its having been autumn in Eden. There's something very 17th-century, very characteristic of Donne and Webster and Browne, in the idea, though (cf. Donne's elegy "The Autumnall"); the best work of the period is both rich and somber, like autumn. I have always liked Empson's theory of the basis of this sentiment (in the Pastoral book):
Since the world has grown small, and the sun near its end, we are set free with a sort of cosiness, an irresponsible concentration, to be happy while we may. [...] The belief that the world was soon coming to an end, found in Donne, Chapman, and Sir Thomas Browne, seems important to them and is not easily explained. Samuel Butler the elder laughs at it as based on a passage in Copernicus about two determinations of the distance of the sun, of which the second made the sun nearer, so that we seemed to be falling into it ; this would provide the last fire. I suspect that there was some astronomical pamphlet which they had all read, but Hakewill's refutation of the belief does not put one on to it. At any rate the belief was respectable but not certain and had proofs from astronomy rather than religion.