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. (*1)  See Tobit 12:7, 11.  Judith 2:2.  Wisdom 2:22 (transl. "mystery"); 14:23.  Eccl. 22:22; 27:16, 17, 21.  2Macc. 13:21. (R.V.).  The passage in Judith is remarkable:  for Nabuchodonosor calls his captains and great men together 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 Gr. word for will or counsel is different. (*2)

By the end of the second century A.D. it was used interchangeably with tupos (= type), sunbolon (= symbol), and parabole (= parable).  When we find the Greek word musterion rendered sacramentum 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".

It is evident to all that God has made known His will "at sundry times and in divers manners" (Heb. 1:1, 2).  He also kept certain things secret, and revealed them from time to time according to His purposes and counsels.  Hence the word musterion is connected with several concealed or secret things in the New Testament.

1. It was used of the secrets of the kingdom:  which had been concealed, until the Lord revealed them to His disciples (not to the People) in Matt. 13:10, 11.  It had not before been known that the kingdom would be rejected, and that there would be a long interval between that rejection and its being set up in glory. This was concealed even from the prophets who foretold it (1Pet. 1:10-12).
   
   
2. In Rom. 11 it is used in connection with the duration of Israel's blindness.  That blindness itself was not a secret, for it had been foretold in Isa. 6:9, 10.  But the duration of the blindness was kept a "secret" from Isaiah and only revealed through Paul (Rom. 11:25).
   
   
3. It was used of a fact connection with resurrection, which had never before been made known to the sons of men. The Lord had spoken of it to Martha (John 11:25, 26), but though she believe it, she did not understand that to those who should be alive and remain to His Coming the Lord would be "the life", and they would "never die" (v. 26).  The Thessalonians who "received the word" were not left in ignorance of it (1Thess. 4:13), for the Lord's words in John 11:25, 26 were explained to them.
   
   But in 1Cor. 15:51 the secret was fully and plainly shown; and it was that "we shall not all sleep".  Up to that moment the universal belief had been that we must all die (cp. Heb. 9:27). thenceforward it was revealed and made known for faith that all would not die, but that those who are alive and remain (lit. remain over) unto the Lord's Coming will not die at all (see note 1Thess. 4:15, and cp. Phil. 3:14).
   
   
4. Side by side with these Divine secrets there was the secret of the [foretold] lawlessness (2Thess. 2:7 cp. Dan. 12:4).  It was already working during the dispensation covered by "Acts"; and had the nation repented at the call of those "other servants" of Matt. 22:4 (Acts 2:38; 3:12-26; &c), those secret counsels of "the lawless one" and "the transgressors" would have "come to the full" (Dan. 8:23).  But now they are postponed and in abeyance until the appointed time.
   
   
5. But "the great secret" which concerns us to-day was not revealed until after the close of that dispensation covered by "Acts".  (see Acts 28:17-31 and App. 180 and 181).

In any case, while there is no doubt about the general order of the Epistles, the actual dates are conjectural, and rest only upon individual opinions as to the internal evidence (Ap. 180).  And, after all, Rom. 16:25-27 is not the revelation of the mystery as given in the Prison Epistles, but an ascription of glory to Him Who had at length made it manifest by prophetic writings (not "the writings of the prophets", for it is the adjective "prophetic", not the noun "prophecy" as in 2Pet. 1:20).  Romans and Ephesians are thus brought together as two central Epistles of the chronological groups:  the one ending one group, and the other beginning the next, both being treatises rather than epistles, and both having Paul for their sole author, while in all the other Epistles he has others associated with him.

As to the great secret itself, it is certain that it cannot refer to the blessing of Gentiles in connection with Israel.  This is perfectly clear from the fact that that was never a secret.  Both blessings were made known at the very same time (Gen. 12:3); and this well known fact is constantly referred to in the Old Testament.  See Gen. 22:18; 26:4; &c.  Deut. 32:8.  Ps. 18:49; 67:1, 2; 72:17; 117:1.  Isa. 11:10; 49:6. Luke 2:32.  Rom. 15:8-12.  But the secret revealed in the Prison Epistles was never the subject of previous revelation.

In Eph. 3:5 it is stated to be "now revealed".  This cannot mean that it had been revealed before, but not in the same manner as "now"; because it is stated that it had never been revealed at all.  It concerns Gentiles; and it was "revealed unto His holy apostles (*3) and prophets by the Spirit", that the Gentiles should be joint-heirs, and a joint-body, (*4) and [joint] partakers of the promise in Christ through the gospel (see Notes on Eph. 3:5, 6). We cannot know the whole purpose of God in keeping this concealed all through the ages; but one thing we can clearly see, viz. that had God made it known before, Israel would of necessity have had an excuse for rejecting the Messiah and His kingdom.

As to ourselves, the question of "who is in the secret?" does not arise. For we are not to suppose that all who do not know of it are "lost".  One thing we know, and that is : it is made known for "the obedience of faith", or for "faith-obedience" (Rom 16.26). It is a subsequent revelation; and the question is, do we believe it and obey it by acting according to it?

Abraham had several Divine revelations made to him.  From his call in Gen. 11 he was a "righteous" man. In ch. 12 he believed God concerning His promises of the future.  In ch. 13 he believed God concerning the promise of the Land.  But in ch. 15 God made a further revelation concerning the see which He would give him; and it is written, "Abraham believed in the Lord, and it was counted (or imputed) unto him for righteousness".  Even so with ourselves and the subsequent revelation of the mystery in the Prison Epistles.  Let us believe it, and we may be sure that it will be counted unto us for something, for some blessing, which those who refuse to believe it will lose.

(*1) In subsequent Revisions of the Sept., Theodotion (A.D. 160) uses it for the Heb. sod (Job 15:8.  Ps.
25:14.  Prov. 20:19).  See notes in loc.

(*2) In Judith 2:2 it is boule (Ap. 102. 4) while in Eph. 1:9 it is thelema (Ap. 102. 2).

(*3) These were not those of the Old Testament dispensation, but were the subjects of a promise by the
Lord Himself in Matt. 23:34.  Luke 11:49, which was fulfilled in Eph. 4:8, 11.  See the notes on these
passages and Ap. 189.

(*4) Greek sussomos, a remarkable word occurring only here in the N.T.