It helps to understand that there is more than one kind of free will.
In The City of God, Book XXII, Chapter 30, Augustine speaks of a "first freedom of will," which consists "in an ability not to sin, but also in an ability to sin"; and also of a "last freedom of will," which is superior, and is not able to sin. The first freedom is a freedom that "man received when he was created upright," whereas the second freedom "shall not be a natural ability, but the gift of God."
In contemporary terms, Augustine's "first freedom of will" is the freedom of choice between good and evil, or between sin and righteousness. Augustine's "last freedom of will" is the state of will that we receive from God when we exercise the first freedom in choosing not to sin, but instead to become a new creature in Christ (see 2 Corinthians 5:17). In that state of will we do not sin, because we have freely given our will over to God, and have therefore received a new will from God. A scriptural source for this is God's words in Ezekiel 11:19; 36:26 that he will remove our heart of stone and give us a heart of flesh instead--the "heart" being a symbol of the human will.
John 8:31-36 records this conversation of Jesus with some Jewish believers:
Then Jesus said to the Jews who had believed in him, "If you continue
in my word, you are truly my disciples; and you will know the truth,
and the truth will make you free."
They answered him, "We are descendants of Abraham and have never been
slaves to anyone. What do you mean by saying, 'You will be made
Jesus answered them, "Very truly, I tell you, everyone who commits sin
is a slave to sin. The slave does not have a permanent place in the
household; the son has a place there forever. So if the Son makes you
free, you will be free indeed."
Putting this together with Augustine's analysis we can conclude that there are at least three kinds of free will, only two of which are truly free:
- Freedom of choice
- Freedom to engage in evil and falsity
- Freedom to engage in goodness and truth
Freedom of choice is a real freedom, because it makes it possible for us to freely choose to follow God's will, which involves engaging in goodness and truth from God.
Freedom to engage in evil and falsity feels to us like freedom when we are engaged in it, but it is actually slavery to sin and therefore to the Devil and hell.
Freedom to engage in goodness and truth is a real freedom, and a greater freedom than freedom of choice, because this is the freedom given to us by God when we choose God and righteousness over the Devil and sin.
In our life here on earth, we are engaged in the first freedom of will, which is the freedom to sin or not to sin. If we choose not to sin during our life on this earth, but to turn our will over to God and become new creations in Christ, then we enter into the last, and greater freedom, which is the freedom to express the goodness and truth of God in our lives. This is the freedom of will that angels have.
This view of freedom is expressed in the famous offer of life or death given by God to the ancient Israelites in Deuteronomy 30:11-20. Though to the Israelites "the land you are entering to possess" was the earthly land of Canaan, Christians have long interpreted Canaan spiritually as the kingdom of God or the kingdom of heaven.
Theoretically, even in heaven we could sin. After all, Job 15:15 says, "God puts no trust even in his holy ones, and the heavens are not clean in his sight." In other words, no one but God is good, as Jesus said (Mark 10:18; Luke 18:19). Even in heaven we are not clean compared to God. We are always being perfected in God's image, but we never become perfect. Perfection belongs to God alone.
However, people in heaven have given their will over to God, and they no longer sin. The very thought of sinning causes horror in their minds and hearts. Because they have given their will over to God, God continually purifies them and withholds them from any human desires to sin, and they willingly and with joy follow God in not sinning. Because they have given their will to God freely, they are still exercising their free will.
Short version: Here on earth we exercise the first freedom granted to us by God, which is the freedom of choice between good and evil. But once our life on earth is over, we will have already made that choice. Then our freedom becomes a freedom to live according to the choice we have made on earth. If we have chosen evil over good, we will become abject slaves in hell. But if we have chosen good over evil, we will attain a greater form of free will, which is the God-given gift and ability to freely live according to what is good and true from God.
P.S. Since someone is likely to ask: This answer is written from a perspective based on the Bible as understood in light of the theology of Emanuel Swedenborg (1688-1772).