Free will is the concept that we make our own decisions. A science-based philosophy (Scientific Image) does not believe in a free will. All things that a person does is driven by something, usually a chemical response within your body.
If you accept that there is no god, no soul and no afterlife, the major pullback of the scientific image is that it seems to "dehumanize" us by making the actions that we make simply controlled by our bodies or deficiencies in the brain.
However, the concept of science "dehumanizing" us is laughable due to the very nature of the scientific image. Its purpose is to recognize what it actually means to be human; fully human: fully animal. To be human is to have a brain, body, intelligence, chemicals that make us work. That is what humans are.
Though with the scientific image, things are not black and white like perennial philosophy teaches free will and involuntary forces of nature. While this is true, I believe the most realistic image is that of a spectrum of the choice your brain (you) makes and the signals that your body, chemicals, or social queues give off.
Suppose you (do not know you) have a potassium deficiency and you get an urge to eat a banana. Is it you that's making the decision to eat a banana, or is it your body's (for lack of a better phrase)?
What if a person gets drunk and starts a fight. The scientific image would say that it wasn't up to a "who" that made the decision but a "what". The "what" is the alcohol. Though one might argue that the decision to drink in the first place is the one at fault. But what if they are genetically predisposed to consumption of alcohol? Can they really help themselves?
This is not to say that that person is not accountable for his or her actions. However, in some situations such as schizophrenia or other severe mental illnesses, this may be the case.
How much is human nature governed by rational agency, or governed by culturally and socially embedded nature over time?
The shortcoming of religion is that it lies to its followers to ease them into comfort. There is no need to lie to people to help them be comfortable with being an animal. There is nothing wrong with being an animal. We still exist on top of the food chain. We still have intelligence and sentience. We are animals; we are complex animals capable of great things.
In an advanced civilization such as the one we exist in now, we need to have accountability for our actions with an understanding that we have at least a little bit of choice in the decisions we make. We need to know that sometimes chemicals will try to make choices for us (obvious personification), but that we often do have a choice to make.