God knows what will happen, but He does not determine what will happen; He knows the future, yet He does not influence it. People make their own choices and God knows what they will decide. One cannot actually cause harm to another through his choices, as the harm that is to be brought upon that person has already been decreed by God. When an evil person kills or steals, it is due to his choice to act in an evil way. Nevertheless, even if he chooses not to engage in a particular act, the same damage will be brought about by one of God’s many emissaries.

God knows what a person will choose, but does not compel him to do so:

Perhaps more may be said: If He knows what will occur before it occurs, then He already knows that a person will disobey Him, so it is impossible for that person to not disobey Him, for His knowledge is perfect…but we must consider that He knows the truth of what will happen. He knows the causes, and when something is caused, He already knows that it will be caused; and when it is caused by a person’s choice, He already knows that that will be the person’s choice.

It is important to distinguish between the person doing the deed, and its outcome:

It is further asked: When an individual, one of God’s creatures, is condemned to be killed, whether as a punishment for sinning or as a [divinely decreed] ordeal, then when an evil person kills that individual, as Jezebel did to some of the prophets, what can we say about this deed, to whom do we attribute it? We conclude that the death is an act of God, while killing is the act of the evil person. If God’s wisdom determined that someone deserves death, then if it had not been carried out by the evil person, the individual would have died from another cause. Likewise, this question is asked with regard to a thief, as financial loss is a decree of the Creator, either as a punishment or as a [divinely decreed] ordeal. How can we say, with regard to a theft, that it is an act of the Creator? The answer is that the loss is an act of the Creator, whereas the theft is the act of a person. If God’s wisdom decreed the loss of a certain item, then if the thief had not stolen it, it would have been lost in a different way. (Rav Se’adya Gaon, Emunot VeDe’ot 4)

Although nothing occurs in this world that is in opposition to God’s will, human beings are able to do what they want, because God decided that we have free will. Therefore, when a person does as he chooses, this is God’s will. This is not to say that a person’s ability to choose impairs God’s foreknowledge to any degree, since God’s knowledge is not external to Him; rather, it is an intrinsic knowledge whose essence we are unable to comprehend.

God wants a person to be able to act in accordance with his own will:

Do not wonder and say: How is it that a person can do anything he desires and be in control of his actions, if nothing in this world can be done without the permission of his Master, and against His will?… Know, that everything that people do is in accordance with His will, even though our actions are in our hands. How? Because the Creator desires that fire and wind rise up, that water and earth descend, that the sphere rotate circularly, and that the other creatures in the world behave in accordance with the nature He desired for them. So too, He desires that a person be autonomous and that he have control over all of his actions, that nothing compel or pull him, but rather, that he, on his own initiative and with his God-given understanding, will do anything that people are able to do.

God’s knowledge is a spiritual knowledge, and we have no way to comprehend it:

Lest you say: Doesn’t the Holy One, blessed be He, know everything that is going to occur; and before it occurs, doesn’t He know that this person will be righteous or wicked? Or does He not know? If He knows that one will be righteous, it would be impossible for him not to be righteous. If you say that He knows that one will be righteous, yet it is possible that he will be wicked, this would mean that God does not have full knowledge of the matter. Understand, that the answer to this question is longer than the length of the land and broader than the width of the sea, and some major [theological] principles are connected to it. But you must understand what I say here…. The Holy One, blessed be He, does not know with a knowledge that is external to Himself as human beings do, for they and their knowledge are two entities. Rather, He and His knowledge are one, and human understanding cannot fully comprehend this. Just as a human does not have the ability to discover or to fathom the true [essence] of the Creator…so too, he does not have the ability to discover or to fathom the Creator’s understanding…. That being the case, we do not possess the ability to know the way in which the Holy One, blessed be He, knows all creatures and all deeds. Nevertheless, we know beyond a doubt that a person’s actions are in his own hands. The Holy One, blessed be He, does not lead him [to act in a particular way], nor does He decree upon him that he must do so. (Rambam, Mishneh Torah, Hilkhot Teshuva 5:4–5)

According to our understanding of the world, we have free will. We are given this understanding so that we will worship God of our own accord. But in truth, everything is in the hands of Heaven.

On the deepest level, everything is in the hands of Heaven. A person’s free will is as [inconsequential as] a garlic peel, and it is only what appears to him to be the case. For God conceals His ways from human beings because He desires them to worship Him, and if everything were revealed before a person, worship would not come from within him. (Rabbi Mordekhai Yosef of Izhbitze, Mei HaShiloaĥ, Koraĥ)

Further reading: The principle of free will appears in the Bible: see A Concise Guide to the Torah, pp. 463, 503.