NET

25 Now to him who is able to strengthen you according to my gospel and the proclamation of Jesus Christ, according to the revelation of the mystery that had been kept secret for long ages,

Darby

25 Now to him that is able to establish you, according to my glad tidings and the preaching of Jesus Christ, according to [the] revelation of [the] mystery^D8195, as to which silence has been kept in [the] times of the ages,

Darby Bible Notes

mystery - It formed no part of what was unfolded in the ages of time, in which God developed his plans in creation; it was a purpose before, and eternally hidden; it was not in reckoned time. See 1Cor. 2.7-10; Eph. 3.2-11; 5.32; Col. 1.25-27; 2.2,3.

KJV

25 Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,