Consider this sentence,
Until this is proven wrong, we will believe it to be true.
Does it mean either;
So, when someone use until like this, does it mean the mentioned thing will happen or might happen ?
Until simply means up to the point in time when. So your sentence could be rephrased as "Up to the point in time when this is proven wrong, we will believe it to be true."
Any certainty or uncertainty derives from the context and the implications of the sentence and not from the use of the word until.
I could say, "Until the subway is completed we will take the bus." From this context it is easy to infer that we believe the subway will be completed and when it is, we will start using it.
The other thing to point out here is that in your example sentence I would expect to hear emphasis on the word proven. In other words I may acknowledge that many people believe it to be incorrect, but until it is actually proven I will continue to believe it.
If the sentence is taken literally, either interpretation could hold. However, for most readers or listeners, only the second interpretation is ever thought of. That is, all but a tiny fraction of auditors take the meaning to be as below, where X denotes the thing in question which is not yet proved or disproved:
We believe X at present, and will continue to do so, until such time (if any) as X is disproved.
Under the second interpretation, there is no implication that X will or might be proved wrong, but such proof is not thought impossible. The first interpretation, on the other hand, seems to require belief that X will be proved wrong. But if you know or believe X will be proved wrong, it is willful perversity to meanwhile believe in it; so apparently the first interpretation is unlikely to ever be correct.
Until this is proven wrong, we will believe it to be true.
Until is unconcerned with whether the associated condition will be true, will not be true, probably/possibly will/will not be true, or will never be true. Another way of reading the OP's sentence is:
As long as this is true, we will believe it to be true.
Certainty/uncertainty is determined entirely by the condition rather than by the conjunction :)
