Summary from "Pocket Handbook of Christian Apologetics" -Kreft and Tacelli
Free will vs determinism... determinism= heredity + environment; free will adds a 3rd cause to our actions= our wills.
Heredity and environment condition our acts, but they do not determine them. They are necessary causes but not sufficeint causes of freely chosen acts.
The simplest argument for the existence of free will is observation of how we use words. We praise, blame, command, counsel, exhort, moralize to each otther. We do not hold machines morally responsible for what they do. If there is no free will, all moral meaning disappears from life.
Why did God give us free will and allow us to misuse it? The ? is misleading. One does not give free will to a human. Free will is part of our essence. There can be no human being without it. The alternative to free will is not being a human but an animal or a machine.
Omnipotence: Evil's source is not God's power but man's freedom. Then why didn't God create a world without human freedom? Because that would have been a world without humans, a world without hate, but also without love. Love proceeds only from free will. Animals cannot love, they can only like.
Isn't a world with free human beings but no sin possible? Yes. And God created just such a world. But such a world - a world in which no-sin is freely possible- is necessarily a world in which sin is also freely possible. And if there are human beings at all, that is, creatures with free will, then it is up to their free choice whether that possibility of sin is freely actualized or not.
Even omnipotence could not have created a world in which there was genuine human freedom and yet no possibility of sin, for our freedom includes the possibility of sin within its own meaning. All things are possible with God, but a meaningless self-contradiction is not anything. You cannot have the possibility of freely choosing good or evil and at the same time no possibility of choosing evil. This is like asking why God didn't create round squares.
God's perfection is His consistency/not contradicting Himself. The logical law of non-contradiction applies to God. It is a universal, eternal, objective truth about reality. We discover, not invent it.
God could forcibly prevent sin (which He does not), but not without removing our freedom.
Lewis: Nonsense does not cease to be nonsense when we add the words 'God can' before it.
No Scriptures? These principles are consistent with God's revelation on every page of Scripture and our personal experiences.