To this end we always pray for you, that our God may make you worthy of his calling and may fulfill every resolve for good and every work of faith by his power. (2 Thessalonians 1:11)

Seeking the power of God to fulfill our good resolves does not mean that we don’t really resolve, or that we don’t really use willpower.
The engagement of God’s power never takes the place of the engagement of our will! The power of God in sanctification never makes us passive! The power of God engages itself beneath or behind and within our will, not in place of our will.

The evidence of God’s power in our lives is not the absence of our willing, but the strength of our willing, the joy of our willing.
Anyone who says, “Well, I believe in the sovereignty of God and so I will just sit back and do nothing” does not really believe in the sovereignty of God. For why would someone who believes in God’s sovereignty so blatantly disobey him?

When you sit back to do nothing, you are not doing nothing. You are actively engaging your will in a decision to sit back. And if that is the way you handle sin or temptation in your life, it is blatant disobedience, because we are commanded to wage a good warfare (1 Timothy 1:18) and resist the devil (James 4:7) and strive for holiness (Hebrews 12:14) and put to death the sinful acts of the body (Romans 8:13).

Second Thessalonians 1:11 says that it is by the power of God that we will fulfill our good resolves and our works of faith. But this does not nullify the meaning of the word “resolve” and the word “work.” Part of the whole process of walking worthy of God’s call is the active engagement of our will in resolving to do righteousness.

If you have lingering sin in your life, or if you keep neglecting some good deed just because you have been waiting around to be saved without a fight, you are compounding your disobedience. God will never appear with power in your will in any other way than through your exercise of that will; that is, through your good resolves — your good intentions and plans and purposes.

So, people who believe in the sovereignty of God must not fear to engage their wills in the struggle for holiness. “Strive to enter through the narrow door. For many, I tell you, will seek to enter and will not be able” (Luke 13:24). Only strive in the faith that in and through your striving God is at work to will and to do his good pleasure (Philippians 2:13).
– John Piper