The leading spokesman against free will and religion today is probably Sam Harris. He has been refining his free will argument for about ten years, as he wrote a 2012 book on the subject. He has now posted a podcast with his best argument against free will. If you prefer to read, it is summarized here.
He has a few omissions.
Religion. Harris was a Jewish atheist who converted to Buddhism, and is very negative about Christianity and Islam. This colors everything he says, even though he does not directly mention the Christian notion of free will.
Drugs. Harris spent his college years taking hallucinogenic drugs, and these seem to have permanently altered his brain. He talks about it in other podcasts. The drugs appears to have robbed him of some mental abilities.
Hypnosis. About 5% of the public is highly suggestible to hypnosis, and Harris has described himself as being one of those. That means that he is the type of person that stage hypnotists use to perform silly stunts to show off a lack of will. Harris is also a big believer in using his podcasts to hypnotize his listeners to follow his thinking. He does this in both his "Making Sense" podcast, where he advocates political and other opinions, and his "Waking Up" podcasts, where he more directly asks the listener to meditate on what he says.
Politics. His political views can best be described as Trumpism while denouncing Trump personally and pretending to be a leftist. Harris is anti-Islam, anti-immigration, pro-gun, pro-police, and most all, vigorously against the excesses of the Left. But he seems to lack the free will to say he supports Trump, and has had many podcast guests who severely denounce Trump personally. He appears to mainly associate with Leftists, and not understand right-wingers at all.
Science. Harris denies that is just making a philosophical argument, but he omits any arguments for or against free will from physics, biology, or neuroscience.
Harris's main argument is that when he appears to make a choice, such as naming a favorite movie, he does not feel as he is freely making the choice. It is all determined by voices in his head, or some such mechanism. He feels no sense of self, and always feels like a slave to external forces.
Of course he ends with a pitch for money, and a promise for more enlightenment if you send money. He denies that he is contradicting himself by asking us to choose to donate, because he says that we will have no choice but to do what is right. Furthermore, he says that it is liberating to a slave to reason and logical persuasion, because then you will be a rational being all the time.
If you are still reading this, and had not heard of Sam Harris, you must think that he is some sort of con man.
I believe him. I think he genuinely lacks the free will that most people have. I don't know if he lost it from drugs, or Buddhism, or Leftism, hypnosis, or what. Maybe he never had it. Listening to him is like listening to an advanced AI automaton.
The closest he comes to a scientific argument is where he repeatedly refers to randomness. He has an extremely determinist world view, but obviously someone told him that quantum mechanics is an exception. So he qualifies himself by saying things are determined except for quantum randomness. But he strenuously argues that free will is supposed to come from the self, and a pre-programmed computer cannot be the self, and the random actions of some quantum particles cannot be the self either.
Being a determinist is tough, because we have no scientific theories that are 100% determinist. There are zillions of scientific papers where the outcomes are 95% explained by the data, but never 100%. A determinist might argue that the papers might get 100%, but for experimental error. If determinism were really false, and assuming reductionism, then it ought to be possible to find experiments with purely unpredictable outcomes. Quantum mechanics provides exactly that.
So Harris's argument is like saying human consciousness does not exist because it depends on an autonomous mind, and the brain is made of atoms, and atoms are not conscious. Not much of an argument.
The physics argument is not his main argument. I mention it here because it is the only part that relates to science.
Harris has mostly made his reputation attacking Islam and Christianity, and espousing atheistic rationality. He talks about morals a lot, and pretends that he is taking the scientific rational approach.
I think that attitudes towards free will explain a lot. To most of us, free will is the most directly observable fact about the natural world. And yet our leading Physics popularizers have abandoned it. Scott Aaronson was one of the last holdouts. But now that he has adopted many-worlds theory, he has implicitly dropped free will. After all, a choice causes the world to branch, and each parallel world is equally real, with each copy thinking it made a choice. So no one ever really makes a choice.