You know the sayings: "May The Force be with you." "Turn to the Dark Side." "Use The Force."
But what exactly, um, is The Force?
A Mysterious Force
While being interviewed in 1977, George Lucas said:
"[The Force is] sort of boiling down religion into a very basic concept. The fact that there is some deity or some power, some… force that controls our destiny or works for good and also works for evil has always been very basic in mankind." (Source)
Throughout the original trilogy, the Force was exactly that: a symbol of the religious belief in a higher, destiny-driven power.
In The Phantom Menace, we finally get to see the religion of the Jedi at work, yet the Force remains as mysterious and elusive as ever. With the exception of midi-chlorians—which we'll discuss in a separate section—we don't learn much more than what Obi-Wan told us in the first movie. The Jedi are the guardians of peace in the galaxy, and the Force is an energy that binds the universe together and gives the Jedi its powers.
Done and done.
An Organized Religion
With that said, we do get to see a bit of the religious organization of the Jedi. The central belief of the Jedi is in the Force, and their religious tenets are called the Jedi Code. (Sounds serious.) This code of conduct tells the Jedi how to properly live their lives for the light side of the Force. We don't learn much about the code in The Phantom Menace, but we know it doesn't allow a Jedi knight to take two Padawan learners. The code forbids it for… reasons.
It also remains unclear whether the Jedi worship the Force or only practice spiritual methods to manipulate it. Qui-Gon believes midi-chlorians conceived Anakin and that training him is the will of the Force, suggesting that not to train Anakin and countering said will would be an immoral action.
This shows he wishes to honor the Force's will as sacred (i.e. worship). On the other hand, we never see a scene where Jedi pray to the Force or perform rituals to pay homage to it. Instead, they tend to meditate upon it and use it more as a spiritual tool.
We also learn the Jedi Order is a hierarchical religion. The masters belong to the Jedi Council, a group of Jedis that make decisions for the rest. Jedi knights make up the workforce, and the Padawans are young Jedi who are still learning to sense the Force and how to live by the code.
The Jedi hierarchy presents us with one aspect of the Jedi Order that hints at the religion's darker tendencies. Most religions allow people to convert of their own free will, giving outsiders the opportunity to adopt their beliefs and doctrines. But the Jedi don't allow converts.
As Qui-Gon says, the Republic identifies Force sensitive children at a young age. These children are then taken to the Jedi and indoctrinated into their monastic way of life. Once they become too old to be properly indoctrinated, as noted by Obi-Wan and Mace Windu, they are turned away from the order and denied training.
Ugh. Now that we really think about it, the Jedi sounds kind of cult-y. That's pretty creepy, Yoda.
Lucas said he boiled down religion to "a very basic concept." But religions are incredibly diverse—a basic concept in one religion might not have a corollary in another. For example, monotheism (the belief in one God) is a cornerstone of Islam, Judaism, and Christianity… but Hinduism states that there are 300 million gods. (Source)
And although the Force has no one-to-one real-world counterpart, it does draw inspiration from several aspects of real religions.
The Tao of Lucas
The Force borrows several qualities from the concept of qi (sometimes spelled "ki" or "chi"). Qi is an idea associated with Asian religions and philosophies, including Taoism and Buddhism. Essentially, qi is an energy that flows through the world and animates it. (Source)
With that said, Eastern religions recognize many types of qi, including masculine, feminine, and ancestral, but the Force is always mentioned as a singular entity. Also, qi is a life force, meaning it doesn't have a will or consciousness.
The Force, however, has a direct will it uses to influence the destiny of the universe. Check out how Qui-Gon notes this when he says,
"Finding [Anakin] was the will of the Force. I have no doubt of that."
This aspect of the Force belongs more to a sentient God than a life energy force.
The Taoist ideal of Wu Wei also serves as an inspiration for the Force, especially Qui-Gon's personal Force philosophy. Wu Wei is translated as "non-doing," "non-action," or "living by or going along with the true nature of the world - or at least without obstructing the Tao - letting things take their natural course." (Source)
Qui-Gon's advice to Anakin parallels Wu Wei:
"Remember, concentrate on the moment. Feel, don't think. Use your instincts."
Like the idea of "non-action," Anakin shouldn't try to race. His actions should come from an effortless acceptance of the situation and his body's own rhythms (read: instincts).
Dark vs. Light
The Force also draws its good versus evil dualism from various religions, including Manichaeism, Zoroastrianism and Christianity. In Manichean cosmology, for example, the physical world is painful and evil and the spiritual world is full of goodness and light. By acquiring spiritual knowledge, one can free his or her soul from the evil, physical world. You can definitely see the parallels of light side/dark side of the Force here. (Source)
The Sith also draw upon the idea of a cosmological struggle between good and evil. In Christian mythology, angels who attempted to nab the throne of God with the Big Bad Lucifer were cast out of heaven to become demons. (Source)
In the same way, Jedi who fail to live up to the code and seek prideful goals—such as power, power, and more power—become Sith. The comparison is most pronounced in The Phantom Menace— the Sith characters actually look like demons. Darth Maul looks like the devil himself, with that red skin and those horns, and Darth Sidious looks like a personification of Death. Ew.
So while the Force has its roots in many real-world religions, it doesn't draw enough from any one for us to say it is the Star Wars equivalent of, say, Catholicism or Hinduism. This allows it to be broad enough of a symbol to allow for many different interpretations, allowing viewers to find their personal Force… and letting people of all different belief systems nerd out and say to one another, "May The Force be with you."