I'm pretty sure this is one of those "too good to be true" quotes. I can find nothing even vaguely resembling it in a Google Books search restricted to the 19th century; the 1810 Encyclopædia Britannica
article on Collins (on this page
, if Google will let you see it) presents a convincing portrait of his character (confirmed by other sources) that makes such snideness unlikely ("His curious library was open to all men of letters, to whom he readily communicated all the assistance in his power; he even furnished his antagonists with books to confute himself, and directed them how to give their arguments all the force of which they were capable. He was remarkably averse to all indecency and obscenity of discourse; and was, independent of his scepticism, a sincerely good man."), and the long article on Clarke has (on p. 178
) a detailed account of the "existence of God" controversy that surely would have mentioned the remark had it been known (it says of Collins: "But this controversy did not stop here, for the celebrated Collins, coming in as a second to Dodwell, went much farther into the philosophy of the dispute, and indeed seemed to produce all that could possibly be said against the immateriality of the soul, as well as the liberty of human actions. This enlarged the scene of the dispute, into which our author entered, and wrote with such a spirit of clearness and demonstration, as at once showed him greatly superior to his adversaries in metaphysical and physical knowledge, and made every intelligent reader rejoice, that such an incident had happened to provoke and extort from him that plenty of strong reasoning and perspicuity of expression, which were indeed very much wanted upon this intricate and obscure subject."). I'm guessing some twentieth-century skeptic thought it up and put it in Collins' mouth, figuring nobody would examine the attribution too closely.