Companion Bible - Appendix 193 - "MYSTERY" (from Appendix 193 of The Companion Bible compiled by E. W. Bullinger)
The English word "mystery" is a transliteration of the Greek word, musterion, (it is from mueo - to initiate or admit to secrets; and mustes was used of the person so initiated), which means sacred secret. It occurs in the Septuagint Version (280 B.C.) nine times as the equivalent for the Chaldee, raz in the Chaldee portion of "Daniel," which means to conceal ; hence something concealed that can be revealed, viz, in Daniel 2:18, 19, 27, 28, 29, 30, 47 and 4:9.
It occurs frequently in the Apocryphal books; which, though of no use for establishing doctrine, are of great value in determining the meaning of Biblical usage of Greek words. In these books musterion always means the secret of friends, or of a king, &c. (In subsequent Revisions of the Sept., Theodotion (A.D. 160) uses it for the Heb sod - Job:15:8, Psa. 25:14, Prov. 20:19.) See Tobit 12:7, 11, Judith 2:2, Wisdom 2:22 (translated "mysteries"); 14:23, Ecclus. 22:22; 27:16, l7, 21, 2 Macc. 13:21 (R.V.). The passage in Judith is remarkable: for Nabuchodonosor calls his captains and great men just before entering on a campaign, and "communicated with them his secret counsel," lit. "the mystery of his will." This is exactly the same usage as in Eph. 1:9, except that the Greek word for will or counsel is different. (In Judith 2:2, it is boule [Appendix 102.4) while in Eph. 1:9, it is thelema, (Appendix 102.2)
By the end of the second century A.D. it was used interchangeably with tupos (=type), sumbolon (=symbol), and parabole, (= parable). When we find the Greek word, musterion rendered sucramentum in the Latin Vulgate of Eph. 5:32, it is clear that it was used as meaning a secret sign or symbol, and not in the modern meaning put upon the word, "sacrament," i.e., "holy mysteries."