What does the Bible say about free will? Why do we have it? Is every decision we make already accounted for in God’s plan for us? To what extent are our lives affected by our free will versus God’s eternal plan?
The discussion of God’s sovereignty versus man’s free will has entangled philosophers and theologians in endless debate for centuries (millennia?). So, we will just focus on your question considering the condition of the believer.
The will is a characteristic of the person. For example, the Lord as man in the garden could say “not my will but thine be done” (Lk. 22:42b). He, the perfect Man, would submit His (human) will to the will of God. This, of course, is the pattern for us. That our will is “free” is shown by the capacity to submit to God. Otherwise, we would be robots.
So in one sense I am free to “do what I want.” The question is, what do I want? As fallen creatures, our desires have been corrupted by sin. This ruin is profound, as Romans 8:7 tells us: “Because the mind of the flesh is enmity against God: for it is not subject to the law of God; for neither indeed can it be” (JND). But, because we are born again and have eternal life, we struggle, as described in Romans 7. What we need is power to “walk in the Spirit,” as Romans 8:1–4 describes. This comes from our obedience to and fellowship with Christ. (As the Sunday School song says, “Read your Bible, pray every day and you’ll grow, grow, grow.”) Our will becomes coincident with God’s will for us. The way is really simple, so simple we miss it by “overthinking” it. The key truth is simply, God is good. The more deeply we realize this, the more simply we commit to His will. As the Lord puts it, “if your eye is healthy [or “clear”], your whole body will be full of light” (Mt. 6:22). The “clear eye” is one that is focused on Christ and His glory. Then, the “whole body will be full of light”; in other words, we will understand the blessedness of His will for us.
Consider John 16:4–14. Here the work of the Spirit in us is described. We often don’t appreciate how the Spirit can lead us without “directing” us. Imagine a friend who walks alongside to give guidance and encouragement. In such a case, I am “free” to accept or reject my friend’s advice. Yet, to reject it reveals a self-will that is in opposition to God’s perfect will. This may result in God’s discipline (see Heb. 12:1-11). But notice that the consequence of this discipline is the “peaceful fruit of righteousness” (v. 11b). There is real peace when we realize the goodness of God and His work in our lives on our behalf. This, it seems to me, is how God’s eternal plan for us is realized in the context of our free will.