"so that no human being might boast in the presence of God. And because of him you are in Christ Jesus, who became to us wisdom from God, righteousness and sanctification and redemption, so that, as it is written, “Let the one who boasts, boast in the Lord.” - 1 Cor 1:29-31
“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, for it is God who works in you, both to will and to work for his good pleasure.”
There is no doubt that the Bible teaches that God works in us, and we work. (Phil 2:13). And it is certainly true that good works may be described as a cooperation of sorts, but (and here is the kicker) good works are not the same thing as sanctification.
Sanctification is something God does TO you, and IN you. He is conforming you to the image of Jesus. Good works are something WE do, but they are only the result of God's work -- part of the outworking of the sanctification of the Spirit. You do not sanctify yourself (even partly): That is the work of the Spirit and His work alone. Our working and obeying is not an effectual cause of our sanctification. We don't make ourselves Holy. If sanctification were synergistic we would literally be contributing a portion of the grace and power which sanctifies us. This would mean we would (at least partly) sanctify ourselves, while the Holy Spirit does the rest.
Consider another work of the Spirit: regeneration, Reformed theologians all agree with the Bible that when the Spirit regenerates a sinner, the fruit of His regenerating work is faith in Christ (John 6:63, 65). The sinner does not cooperate in order to be regenerated. The work is monergistic which means he is utterly passive in regeneration under the Holy Spirit through the preaching of God's word. His faith (and his willingness to believe) exists in him only after the Spirit disarms his hostility and renews his heart. We do not consider faith to be cooperating with regeneration but the fruit of the Spirit's regeneration.. Likewise, for sanctification, the Spirit sanctifies a sinner, and the fruit of sanctification is good works, but the sinner does NOT cooperate with God IN ORDER TO BE sanctified. Good works flow from our union with Christ, and His sanctifying work in us, but do not contribute to it in any way, since sanctification is part of our salvation - Christ's work.
So sanctification is monergistic.
Don't get me wrong, we cooperate in the Christian life as our regenerate, sanctified hearts yeld to God working in us. Indeed we are obligated to cooperate with God working in us and do good works. We take very seriously the divine commands to obey to do good. But the good works themselves aren't to be confused with sanctification. Sanctification is a work that precludes all good works. We work FROM our sanctification. We don't' work FOR our sanctification. We are passive in sanctification, but active in good works.
The Westminster Confession says it like this:
Q. 35. What is sanctification?
A. Sanctification is the work of God’s free grace, whereby we are renewed in the whole man after the image of God, and are enabled more and more to die unto sin, and live unto righteousness (Shorter Catechism).
Q. 75. What is sanctification?
A. Sanctification is a work of God’s grace, whereby they whom God hath, before the foundation of the world, chosen to be holy, are in time, through the powerful operation of his Spirit applying the death and resurrection of Christ unto them, renewed in their whole man after the image of God; having the seeds of repentance unto life, and all other saving graces, put into their hearts, and those graces so stirred up, increased, and strengthened, as that they more and more die unto sin, and rise unto newness of life (Larger Catechism).
…the principal acts of saving faith are accepting, receiving, and resting upon Christ alone for justification, sanctification, and eternal life, by virtue of the covenant of grace. Westminster Confession of Faith 14.2
NOTE: Now I know Dr. Sproul and others have taught that sanctification is synergistic. But it is pretty clear that what they mean by this is that the Christian life is not passive and we DO obey and work. We are not quietists. But a careful reading will reveal that he does not mean that we sanctify ourselves.
Is Sanctification Monergistic or Synergistic? A Reformed Survey by Kevin DeYoung
Is Sanctification “Synergistic”? by William Evans
Is Sanctification Synergistic? by Jim Mclarty