μυστήριον mystērion

Something known to an initiate: a mystery or secret doctrine, often in the plural. Later writers use it of any secret, not of what is intrinsically hard to understand. In the New Testament it names the counsels of God, once hidden and now revealed in the gospel or in some fact of it, and also particular details of that revelation. Rom 11:25 uses the singular of one such particular disclosure (τὸ μυστήριον τοῦτο). What that disclosure asserts is a question of context, not of the lexicon entry alone.