What is a Christian's responsibility in sanctification?

Marcus Serven explains in this video that one of the key passages in Scripture that explains and discusses the interplay between holiness and knowing that you are complete in Christ is found in Philippians.

While it may seem as though you alone are responsible for your sanctification, Paul in the book of Philippians explains that while you do indeed make effort, God also works in you for His good pleasure. While justification is a single, monergistic effort in that only God changes you and declares that you are no longer under his judgment, sanctification is synergistic in that Holy Spirit is at work in you while you cooperate in working out your salvation in fear and trembling. Thus, you are making efforts as you are being convicted by your sin. These take human decisions and resolution, but God is also at work in you. Thus, sanctification is of a cooperative nature.

Philippians 2:12-13 (NKJV) - “Therefore, my beloved, as you have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling; for it is God who works in you both to will and to do for His good pleasure.”

One of the key texts that talks about this interplay between holiness and also knowing that you are complete in Christ is this one from Philippians chapter 2 where Paul says in Philippians 2, 13, actually 12 and 13, "'Therefore, my beloved, as you have always obeyed, so now, not only as in my presence, but much more in my absence, work out your salvation with fear and trembling. For it is God who is at work in you, both to will and to work for his good pleasure." And the interesting dynamic in that text is that here you have Paul making an exhortation that you're to work out your own salvation. And if that's all he said, you get the impression that somehow it's all up to you. You know, you have to make the effort here and you have to grow, you have to reform yourself, you have to change yourself, you have to somehow put out the old man and put on the new man, all those kinds of scriptural admonitions. But he goes on to say that not only do you make efforts, but God is at work in you, both to will and to work for his good pleasure.

So that's the wonderful dynamic of sanctification here. You work and God works. And so in that respect, whereas justification is a single effort, it's monergistic in the sense that only God does it. He changes you and declares that you are no longer under his judgment. In contrast, sanctification is synergistic in the sense that the Holy Spirit is at work in you, but you're cooperating in this whole effort.

You're working out your salvation in fear and trembling. It's an interesting tension point in sanctification that you're making efforts as you're convicted of your own sin, you're resolving, that you're going to change your speech, that you're going to have different values, that you're going to live in a different way, you're going to use your money differently, you're going to submit your future to the Lord. All of those take human decisions and human resolutions, but God is at work in you, both to will and to work for his good pleasure. And so that brings out the cooperative nature of sanctification, and personally I think that's one of the key texts in understanding sanctification properly.