I take it you're trying to pair off Biblical allusions, "Gospel truth" versus something-Biblical-sounding.
My first thought would be "diabolical lie". "Diabolical" comes from the Latin word for devil, so that would have some theological "feel". And it's a phrase used often enough that you wouldn't have to explain it. (I'm assuming here that by opposite of "Gospel truth" you mean an extreme lie, as opposed to a questionable truth.)
There are plenty of cases in the Bible of people lying: Ananias and Sapphira lying about their generosity, Potiphar's wife making false rape allegations, Abram claiming that Sarai was his sister, Samson and Delilah lying to each other about the source of his strength and why she wanted to know, etc etc. (Pause here to allow atheists to interject that it's all a pack of lies.) I don't know of any stock phrases alluding to such incidents, though. You could always spin a phrase, "You can't trust Mr Jones boasts about his charitable contributions any more than you could trust Ananias" or something like that. If you're looking to be readily understood, such an allusion would simply confuse anyone not familiar with the Bible story. But if you're trying to make more colorful writing, it might be workable.