The Church is intolerant in principle because she believes; she is tolerant in practice because she loves. The enemies of the Church are tolerant in principle because they do not believe; they are intolerant in practice because they do not love.
Edit: I’m leaving the original post below, but thanks to commenter Nicholas Scoville, this quotation has been found and is authentic. It is in God, His Existence and Nature, on page 412 of Vol. II of the 1936 Herder edition.
He seems to think he’s citing a common saying, but since he gives no source, this is probably as far as the trail goes.
Attributed to Reginald Garrigou-Lagrange, O.P., sometimes said to be in his book God, His Existence and Nature. It does sound like something he could have said, but another one of my “fishy quote” triggers is things that fit too neatly into a modern zeitgeist, and this qualifies. In this case, it’s the “defense of orthodox Catholicism” zeitgeist, but it’s a zeitgeist still, so I went looking.
The book it’s alleged to be in happens to be online. I’m not sure that it’s legal–the copyright doesn’t seem to have been renewed, but RGL was a foreign national, which changes things, and I’m not an expert on copyright law–but I went ahead and searched the site it’s on for the words tolerant intolerant practice principle. It’s not there.
RGL does not have a Wikiquote page, so that was no help.
A generalized Google search … I don’t know why I bother. Actually I do know, because a generalized Google search turns up a citation for authentic quotations pretty quickly most of the time. It didn’t for this one.
On to Google Books, which didn’t find it at all, anywhere (which I frankly find hard to believe, but I searched several different ways and nothing turned up).
Back in desperation to a general Google search, using the whole phrase The Church is intolerant in principle because she believes; she is tolerant in practice because she loves. I checked all the likely looking references on the first three pages of the search results, and a few not-so-likely ones as well. No dice.
I’m filing this one as a fauxtation until someone gives me a reference more precise than an entire book.