Through the power of the Holy Spirit, God sanctifies the ones he calls out of darkness into his marvelous light. But we who are called out of darkness and given the Holy Spirit are also responsible for growing in faith and obedience. I appreciate how Robert Letham summarizes this:
Paul writes of us having been predestined to sanctification by the Father (Eph. 1:4-5). Elsewhere he speaks of it being the will of God (1 Thess. 4:3), and of our receiving the Holy Spirit for that purpose (John 17:17, 19; Eph. 5:25, 27; 1 Thess. 4:7-8; 5:23; etc.). Specifically, sanctification is appropriated to the Spirit (Rom. 8:13-14; 2 Thess. 2:13; 1 Pet. 1:1-2). He indwells the people of God. Scripture is silent as to how the Spirit operates, for this is beyond our understanding, as Jesus points out in John 3.
Notwithstanding, sanctification requires our fullest effort as well. It is not a case of sitting back and letting the Holy Spirit take over, for the Spirit works through our own responsible engagement. Here Romans 8:12ff is important to grasp. We are obliged to put to death the deeds of the flesh. However, it is by the Spirit that this is done. Again, in Philippians 2:11-12, Paul writes that we are to work out our salvation; but it is God who puts this desire in us and brings it to effect. Similarly, Paul can say that the grace of God was evident in his life, since he worked harder than anyone else (1 Cor. 15:10).