This is evident early in the film, when the protagonist is forced to sit down and listen to a group of other-worldly agents who explain to him that everything he has ever believed about his own life and choices has been wrong. He has been living under the illusion that he is free. In reality, the important choices he has made in his own life have been carefully directed by a higher and unseen power. In fact, he is told that there are behind-the-scenes powers who keep him on a path he is meant to travel. These forces purposefully cause little things, usually seen by us as mere chance or accident - a spilled cup of coffee, a chance meeting in a hallway, an early bus, a missed cell phone call - that result in life changing consequences.
David Norris (Matt Damon) reels at the prospect that there are even thoughts in his own head that are not his own, planted there by outside powers. His understandable reaction is both a feeling of revulsion and a sense that this is wrong.
"What about free will?" he asks the agents who have been clearly violating it. The answer he is given is that, because of human nature, when man is given free will, man does not usually choose what is good. Evil, pain and suffering regularly result from whenever man is allowed the power to exercise his own will. This is even proved by David's own life, where parties, bar fights and prison have resulted from his own natural tendencies when left unchecked - left to his own devices, David would not be in the position of ability to help other people in need (that he has been purposefully placed in). He would be too immersed in his own self to be capable of channeling his abilities for good or any higher outside purpose. In other words, while violating human free will, the higher power working a design in our lives is a Benevolent power.
And yet ... and this is where The Adjustment Bureau rises another level above regular films ... there is still something wrong with this. David cannot accept the fact that he does not possess free will. David, and later in the story his love interest Elise (Emily Blunt), both rebel against having their wills, and therefore their lives, controlled by an outside power, even if that power is acting for their own benefit. Accepting that choosing to live out their love for each other will hurt them, accepting that being together is going to force each of them to sacrifice personal dreams and desires, and accepting that focusing on each other instead of just their careers is going to severely limit them, they both still decide to be together.
I doubt that it's a coincidence that love is the reason David decides to fight against the forces that are working to force his life towards a predetermined goal. Before you dismiss the film as just another "true love conquers all" or "soul mates" type of romance, consider this: Isn't love a thing that is only possible when we have free will? C.S. Lewis thought so. In Book II, Chapter 3 of Mere Christianity, Lewis wrote:
God created things which had free will. That means creatures which can go either wrong or right. Some people think they can imagine a creature which was free but had no possibility of going wrong; I cannot. If a thing is free to be good it is also free to be bad. And free will is what made evil possible. Why, then, did God give them free will? Because free will, though it makes evil possible, is also the only thing that makes any love or goodness or joy worth having. A world of automata - of creatures that worked like machines - would hardly be worth creating. The happiness which God designs for His higher creatures is the happiness of being freely, voluntarily united to Him and to each other in an ecstasy of love and delight compared with which the most rapturous love between a man and a woman on this earth is mere milk and water. And for that they must be free.
So it's not necessarily that cliched to make a couple's love for each other to be the driving force behind rebellion against predetermined outcomes. While we're on this, let's also settle another question. In discussing this film with a couple friends, a few of them didn't think that the higher power controlling the predetermined outcomes of everyone's lives was necessarily denying them free will. In other words, each person in this film could exercise his or her own will, but outside forces would then just force each person onto a particular path. This explanation just misunderstands the nature of "free will."
I would respond that this understanding is on the level of handcuffing a man to a chair and then telling him that he is free to choose whether to stay or get up out of the chair. Sure, mentally he's still free to make the abstract choice to get up out of the chair, but as long as he's handcuffed to it doing so will be physically impossible. Apply this to David's situation in the film. He may still be mentally free to choose to love and pursue the girl. In that sense you might say he still has the ability to exercise his will. But, he is really not free to pursue and marry her, because outside forces are exercising their powers to make this impossible. This makes any idea of his being free a joke. If you really have free will, you have the ability to choose (and act upon) one of at least two different options.
When the other-worldly agents/angels tell David that he cannot get together with or marry the girl, he is being denied his free will. Another way of putting it is to say that he is being forced to go along with one option against his will. This is a form of slavery. Slavery is being held and controlled against your will. And yet, there are whole number of religions and philosophies that teach precisely that this is the sort of world we live in - that providence/God/the divine being has already predetermined our paths and there is nothing we do can change our fate. This idea should be abhorrent, because it's wrong.
And this is why I like this movie, because our protagonist knows that this is wrong. He's not interested in fine distinctions about how he really gets to keep his free will mentally but just isn't free to physically pursue certain options. He considers being told that he is not free to pursue one option as wrong. Note: there's an important difference between denial of free will and the existence of right and wrong. It would be one thing for a Creator to tell his creatures to follow certain moral rules, and then leave them free to follow them. It would be another thing altogether for a Creator to tell his creatures to follow certain rules and then purposefully mold his universe to make it impossible for them to violate those rules. When David looks in the eyes of the girl of his dreams and says "This can't be wrong," he's not just affirming his own desires against any moral code, he's stating that he believes in a moral code and that the choice he's making does not go against it.
C.S. Lewis explored these further when he argued that the existence of a fixed environment for us to act in was necessary for the existence of free will (in The Problem of Pain, chapter 2):
Again, the freedom of a creature must mean freedom to choose: and the choice implies the existence of things to choose between ... If a man traveling in one direction is having a journey down hill, a man going in the opposite direction must be going up hill. If even a pebble lies where I want it to lie, it cannot, except by coincidence, be where you want it to lie. And this is very far from being an evil: on the contrary, it furnishes occasion for all those acts of courtesy, respect, and unselfishness by which love and good humour and modesty express themselves. But it certainly leaves the way open to a great evil, that of competition and hostility. And if souls are free, they cannot be prevented from dealing with the problem by competition instead of by courtesy. And once they have advanced to actual hostility, they can then exploit the fixed nature of matter to hurt one another. The permanent nature of wood which enables us to use it as a beam also enables us to use it for hitting our neighbor on the head ...
We can, perhaps, conceive of a world in which God corrected the results of his abuse of free will by His creatures at every moment: so that a wooden beam became soft as grass when it was used as a weapon, and air refused to obey me if I attempted to set up in it the sound waves that carry lies or insults. But such a world would be one in which wrong actions were impossible, and in which, therefore, freedom of the will would be void ...
So you see, this is the understanding possessed by both David and the angel, Mitchell. They may be able to mentally exert their will, but they still live in a world in which God corrects the results of the abuse of free will by directing and determining the paths and fates of each person - essentially make the exercise of will void. So while some philosophers and some churches today actually do teach that this is what Divinity does, the filmmakers of The Adjustment Bureau don't agree.
It should be fair to say that the world of The Adjustment Bureau, where agents of deity guide each man's steps toward a predetermined outcome, and where each man or woman is not free to pursue particular goals is a world inconsistent with the teaching of Christianity. Of course, it is true that Christianity teaches that God does sometimes intervene in the affairs of man. Some things are predetermined. But never anything that deprives man of free will.
So once the filmmakers were stuck with this wrong world in their story, what should they have done? They had their protagonist rightly rebel and fight against it. They had different characters question or affirm the way a deterministic world would work. And finally, they reach a conclusion that, if not quite theologically correct from a Christian point of view, at least reflects the fundamental understanding that there is still something very wrong with Determinism. The Adjustment Bureau is essentially the story of a character fighting against Determinism, and by doing that somehow the film turns inspiring.