Jamieson, Fausset and Brown on 2 Timothy 1:9

9. Who ... called us--namely, God the Father (Ga 1:6). The having "saved us" in His eternal purpose of "grace, given us in Christ before the world began," precedes his actual "calling" of us in due time with a call made effective to us by the Holy Spirit; therefore, "saved us" comes before "called us" (Ro 8:28-30).

holy calling--the actual call to a life of holiness. He 3:1, "heavenly calling" [Tittmann, Greek Synonyms of the New Testament]; whereas we were sinners and enemies (Ep 1:18; 4:1). The call comes wholly from God and claims us wholly for God. "Holy" implies the separation of believers from the rest of the world unto God.

not according to--not having regard to our works in His election and calling of grace (Ro 9:11; Ep 2:8, 9).

his own purpose--The origination of salvation was of His own purpose, flowing from His own goodness, not for works of ours coming first, but wholly because of His own gratuitous, electing love [Theodoret and Calvin].

grace ... given us--in His everlasting purpose, regarded as the same as when actually accomplished in due time.

in Christ--believers being regarded by God as IN Him, with whom the Father makes the covenant of salvation (Ep 1:4; 3:11).

before the world began--Greek, "before the times (periods) of ages"; the enduring ages of which no end is contemplated (1Co 2:7; Ep 3:11).