The mystery (το μυστηριον). See on 1. Corinthians 2:7 for this interesting word from μυστης (initiate), from μυεω, to wink, to blink. The Gnostics talked much of "mysteries." Paul takes their very word (already in common use, Matthew 13:11) and uses it for the gospel.

Which hath been hid (το αποκεκρυμμενον). Perfect passive articular participle from αποκρυπτω, old verb, to hide, to conceal from (1. Corinthians 2:7; Ephesians 3:9).

But now it hath been manifested (νυν δε εφανερωθη). First aorist passive indicative of φανεροω, to make manifest (φανερος). The construction is suddenly changed (anacoluthon) from the participle to the finite verb.