Ephesians 3:5

Which in other generations was not made known unto the sons of men, as it hath now been revealed unto his holy apostles and prophets in the Spirit.

The Scriptural definition of "mystery" is apparent here, the mystery being God's plan of redeeming man, once concealed, now revealed.

As it hath now been revealed ... All of the commentaries examined by this writer fail to see the essential limitations imposed by this clause. What Paul said here is not that the present revelation of the mystery is final and complete, but that the previous generations did not possess a revelation of it "as it hath now been revealed." Revelation 10:7 states that the mystery of God will be finished, or "is finished" in the days of the voice of the seventh angel, when he is about to sound; and it will hardly be denied by any that this means it is not finished now! Marvelous as the Christian revelation surely is, there is no ground for people assuming conceitedly that they "know all about it."

Holy apostles and prophets in the Spirit ... Far from claiming to be the unique source of God's revelation of the great mystery, Paul here declared that the "holy apostles and prophets" of the first Christian generation (all of them) were likewise participants in having received from God this glorious revelation. Paul was both an apostle and a prophet; but Paul did not here preempt the title "holy" unto himself; but there was no honorable way in which he could have denied it to that sacred group to which he himself surely belonged. Bruce has a perceptive comment thus:

The reference to the "holy apostles and prophets" has been felt to have an impersonal ring about it, making it difficult to imagine Paul himself writing it; but the difficulty lies rather in our twentieth century English ears than in first century New Testament Greek. There is nothing formal or liturgical about Paul's use of the adjective "holy," and nothing unnatural about the way in which he associates the other apostles and prophets with himself.[15]