Few things get me off (angry) as much as hearing this. In fact, recently, I saw this on a t-shirt - as a quote, supposedly, from Leo-13. First, doing a Google search does not reveal Leo and probably does not reveal any Catholic clergy. I haven't done more than just a cursory search. But I wanted to read the writings of Leo-13 long before I saw this "quote".
Now when reading Immortalie Dei and Libertas there is obviously an encyclical or some writing or understanding between these two. I think it may be St.Augustine's _City_of_God_. And while I still have to read them both several times, as well as City of God, I've come to some understanding.
First, many of those espousing that "Liberalism is a Sin" are either conservative or arch-conservative. And for them, including supporting anti-abortion, including being harsh upon the poor and needy - they all view it as "being Godly" or "being godly".
What they don't realize (and what they probably choose to remain ignorant of) is that Leo wrote during the late nineteenth century. People who claim to be as high and lofty as these should consider that what "Liberal" meant one- or two-hundred years ago is not what we mean by "Liberal" today.
In Libertas, Leo is very clear. It is in the same voice that Satan, through Liberalism, tells God "I will not serve". That is not what modern Liberals mean at all by "Liberalism".
Unfortunately those arch-conservative do not recognize Modertism as being a legitiamate way. Perhaps they consider it "Liberal Lite".
But what they fail to realize is that what Leo meant by "Liberal" is not what they mean by "Liberal". So the quote from Leo or whoever is entirely out of context and *wrong*.