I think you run into the problem of definitions. An idea cannot be absolute, perfect truth without total, perfect, complete definitions for its terms. This isn't required for knowledge - the terms can be rough because the ideas are tentative. But for absolute truth, the boundaries of meaning of your terms must be completely determined. But, as the postmoderns pointed out, this requires infinite information - the complete determination of any one term requires its distinction from all other terms. In fact, they didn't go far enough. I'd argue you would need to know the distinction between the term and all other possible terms.
You have to know perfect definitions in order to have the idea in your head be perfectly true. Perfect definitions require infinite information, therefore you cannot know perfect truth.