To understand a proposition means to know what is the case if it is true.
(One can understand it, therefore, without knowing whether it is true.)
It is understood by anyone who understands its constituents.
~ Wittgenstein, Tractatus Logico-Philosophicus, 4.024
In relation to this particular point, I am not sure whether or not I am entirely in agreement with Wittgenstein. Here is why I think I disagree:
I am inclined to accept Wittgenstein's initial statement that understanding a proposition means knowing what is the case if that proposition is true. However, I have trouble with the next two statements because I am inclined to believe that to know what the case is if any proposition is true requires us to experience something of that case. Or, to translate that into more Christian language, knowledge of the truth is premised upon an encounter with the truth. Thus, I am inclined to argue that there is actually no understanding of a proposition that is not true, for one cannot even understand a proposition's constituents apart from an encounter with the truth, and false propositions cannot provide such an encounter. All false propositions are as comprehensible as the statement, “this circle is a square.” Furthermore, apart from an encounter with the truth, what we imagine the case to be if a certain proposition is true, and what the case actually ends up being if a certain proposition is true, will always end up being radically different.