A possible way of understanding it is,-the point is understanding is always partial, never complete. One argument for this is simply that we all have a limited and subjective window onto the world, - another is, that even simple propositions, contain loads of suppressed information, - complicated propositions pack in a huge amount of info so to fully understand or explain a complicated proposition, you need to supply a whole load of supplementary propositions, - which in turn require further elucidation, - which is potentially a never-ending process. All propositions are a kind of shorthand, because all rely on a supporting framework that we assume without realising.
The supporting framework that we assume without realising may well be wrong and conceptual frameworks may change over time.
So, absolute truth can only really be appreciated by the "mind of God" which sees things from all perspectives and possible conceptual frameworks.
And humanity approaches absolute understanding or becomes more like the universal geist through the process of the dialectic worked out through history. It may sound like arbitrary mysticism, but I think it makes sense.
To ask for evidence for it sounds like a strange question, I don't think hegel offers evidence, he offers argument. If I've understood it correctly, which quite possibly I haven't, as I've never read it, - the argument is something like -
we have a notion of truth, such that true=1 and positive=0
Without this notion being justified, our whole conceptual framework breaks down. '
It's very difficult to justify this notion, as human truths seem to be (at best partial, or) true or false relative to the conceptual framework generally used for evaluating propositions of this sort.
To justify claims of absolute truth, you need not only prove the empirical truth of a proposition, but also that the conceptual framework underlying the proposition is the proper framework.
To do this you have to postulate universal Geist, or else there will be no way of justifying one conceptual framework over another.
That may be all wrong, - but it's how I made sense of Hegel, and I think it kind of makes sense, irrespective of whether it's what hegel meant or not.