Kitto's Popular Cyclopedia of Biblial Literature
A most unscriptural and dangerous sense is but too often put upon this word, as if it meant something absolutely unintelligible and incomprehensible; whereas, in every instance in which it occurs in the Sept. or New Testament, it is applied to something which is revealed, declared, explained, spoken, or which may be known or understood. This fact will appear from the following elucidation of the passages in which it is found. First, it is sometimes used to denote the meaning of a symbolical representation, whether addressed to the mind by a parable, allegory, etc. or to the eye, by a vision, etc. (; ). Again, the mystery or symbolical vision of the 'seven stars and of the seven golden candlesticks' (; ), is explained to mean 'the angels of the seven churches of Asia, and the seven churches themselves' (). Again, 'the mystery' or symbolical representation 'of the woman upon a scarlet-colored beast' () is also explained: 'I will tell thee the mystery of the woman.' etc. (). When St. Paul, speaking of marriage, says, 'this is a great mystery' (), he evidently treats the original institution of marriage as affording a figurative representation of the union betwixt Christ and the church. The word is also used to denote anything whatever which is hidden or concealed, till it is explained. Thus it is employed in the New Testament to denote those doctrines of Christianity, general or particular, which the Jews and the world at large did not understand, till they were revealed by Christ and his apostles, 'Great is the mystery of godliness,' i.e. the Christian religion (), the chief parts of which the apostle instantly proceeds to adduce—'God was manifest in the flesh, justified by the Spirit, seen of angels,' etc.—facts which had not entered into the heart of man () until God visibly accomplished them, and revealed them to the apostles by inspiration (). Thus also, the Gospel in general is called 'the mystery of the faith' (), and 'the mystery which from the beginning of the world had been hid with God, but which was now made known through means of the church' (). The same word is used respecting certain particular doctrines of the Gospel, as, for instance, 'the partial and temporary blindness of Israel,' of which mystery 'the Apostle would not have Christians' ignorant (), and which he explains (). He styles the calling of the Gentiles 'a mystery which, in other ages, was not made known unto the sons of men as it is now revealed unto the holy apostles and prophets by the Spirit' (; comp. 1:9-10, etc.). To this class we refer the well-known phrase, 'Behold I show you a mystery (), we shall all be changed;' and then follows an explanation of the change (). And in the prophetic portion of his writings 'concerning the mystery of iniquity' (), he speaks of it as being ultimately 'revealed' (); and to complete the proof that the word 'mystery' is used in the sense of knowable secrets, we add the words 'Though I understand all mysteries' ().
Kitto, John, ed. Entry for 'Mystery'. "Kitto's Popular Cyclopedia of Biblial Literature". http://www.studylight.org/encyclopedias/kbe/m/mystery.html.