That is an interesting sentence.
Because without belief we do not have faith. And without faith we do not come to Christ. So without belief, faith we do not have God working in us.
Practicly every Calvanist spouts the 'God working in me' every other sentence. It is like the cheap-seats of the doctrine. Not calling it what it is which is works, but by putting God at the front of your phrase, you misrepresent God and yourselves.
Saying it is God working in you, does not make it so. Because works-oriented people always have that excuse.
1 So if there is any encouragement in Christ, any comfort from love, any participation in the Spirit, any affection and sympathy,
2 complete my joy by being of the same mind, having the same love, being in full accord and of one mind.
3 Do nothing from selfish ambition or conceit, but in humility count others more significant than yourselves.
4 Let each of you look not only to his own interests, but also to the interests of others.
5 Have this mind among yourselves, which is yours in Christ Jesus,
6 who, though he was in the form of God, did not count equality with God a thing to be grasped,
7 but emptied himself, by taking the form of a servant, being born in the likeness of men.
8 And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross.
9 Therefore God has highly exalted him and bestowed on him the name that is above every name,
10 so that at the name of Jesus every knee should bow, in heaven and on earth and under the earth,
11 and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.
12 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 own salvation with fear and trembling,
13 for it is God who works in you, both to will and to work for his good pleasure.
Long passage, I know. But it's important to see it all.
When Paul said we should work out our salvation with fear and trembling, he said its God who works in you. There is a "therefore" in there, though, that tends to get overlooked.
In the verses prior, Paul explains how Jesus didn't use His diety to get by. He said He didn't count equality with God something to be grasped (used). So what did He do? He relied on God's power. He humbled Himself. He became a servant. So the "therefore" is telling us that if Christ relied on God (the Spirit), then we should too. That's the fear and trembling and God working in us.