Some time ago I came across a rather odd quotation here.
No matter how sinful one may have been, if he has devotion to Mary, it is impossible that he be lost. – St Hilary of Poitiers.
Now that sounds like a very modern Roman Catholic position, rather than anything ancient. But did Hilary say it?
A couple of Hilary’s surviving works are in the NPNF translation here; On the Trinity, and On the Synods. It’s easy enough to search these for “Mary”, and see if the quote is there. It does not seem to be.
Hilary also wrote a number of letters to Constantius, and some historical stuff – hard to work out what is what -, at least some of which was translated in the Liverpool University Press series as Hilary of Poitiers: Conflicts of Conscience and Law in the Fourth-century Church by Lionel Wickham (1997). There is also a Commentary on Matthew (translated in 2012 in the Fathers of the Church series, preview here), a Commentary on the Psalms (no translation known to me), and a De mysteriis (a French translation exists).
A collection of hymns also exists, edited by W.N.Myers for a dissertation in 1928: The hymns of Saint Hilary of Poitiers in the Codex Aretinus; an edition, with introduction, translation and notes. It was published by the university of Pennsylvania, and is 82 pages long, according to Google Books, who have digitised it but not made it available.
Unfortunately I don’t have access to all these, so I can’t check more than the online material. But little of this seems likely to contain our quote.
Google Books search by date range appears to be broken tonight. But looking through the results, and also in Bing, it looks very much as if this “quote” first appears in the 21st century.
I think we must mark this one as highly suspect. But until we can access the rest of Hilary’s works, we can’t know for sure.