[Even] the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
Note 3 at Col 1:26: The specific mystery to which Paul was referring is explained in the next verse (see note 5 at Col 1:27). When he said the mystery had "been hid from ages and from generations," he was speaking about the generations of people under the Old Testament era. They didn't understand the New Covenant mysteries that were prophesied in the Old Covenant, because these mysteries could not be understood apart from divine revelation (see note 4 at 1Co 2:9, note 5 at 1Co 2:10, and note 3 at 1Co 2:14).
In contrast, the New Testament believer has now had these mysteries made "manifest." The English word "manifest" was translated from the Greek word "PHANEROO," and this Greek word means "to render apparent" (Strong's Concordance). That which was hidden is now made apparent to those who receive the ministry of the Holy Spirit (see note 35 at Joh 14:26).