John Hendryx, at Reformation Theology, posts a good answer to an enquirer concerning the believer’s role in sanctification:
Visitor: I think that conversion and regeneration are mongergistic; but is repentance synergistic, and faith synergistic; is sanctification synergistic?
Response: In a manner of speaking yes. One could say that technically only regeneration is monergistic. This is because it is the only grace God gives us when we are utterly passive, or even hostile to Him. We do not cooperate with God to be regenerated. He regenerates us and, as a consequence of our new heart and the Spirit living in us, we most willingly cooperate. And, like you appear to conclude in your question, it is we, and not God, who actually exercise faith and repentance. Yet, I must admit that I am uncomfortable with the word “synergism” when applying it to faith, repentance and sanctification. This may perhaps be an improper word since these things effectually arise from a renewed heart. We are already redeemed so we are not working toward salvation, but rather, working from out of our salvation. We therefore no longer work in the sense that we are striving to earn anything redemptively. That is already a fait accompli. Our work is, rather, the inevitable outworking of our adoption as sons who are no longer slaves. We work from thankful hearts that spring forth from our new nature which God himself has given us. Having been regenerated, our own work never maintains our just standing before God. Christ has already proved sufficient for this and having joined us to Himself has given us rest from our work (read Heb 4).