The basic aspects of becoming a Jedi are laid out in all six movies. These include a natural aptitude (what is later called a high midi-chlorian count), and a willingness to begin and continue training their mind, body and spirit in the best ways to use the Force (and be used by it) for the good of all.
What is a Jedi
First we need to point out that a Jedi is not simply any Force User. A person sensitive to the Force might be able to enhance their natural skill. But they would have no way of knowing more than that is possible without some kind of training. Even then, it needs to be specific training – not Sith training. A Jedi is more than the Force. A Jedi lives by a certain code. While the Code may have been modified over millennia and subject to different interpretation, the basics of it have remained unchanged. A Jedi must use the Force for selfless ends.
Midi-chlorians
First of all, midi-chlorians are not the Force but are a means to access it. They also aren’t new in concept, in spite of the name first appearing in The Phantom Menace. The notion that the Force has a physical component is implied in Return of the Jedi. Let’s do a bit of comparing between the two trilogies.
The Original Trilogy


The Emperor knew, as I did, that if Anakin were to have any offspring, they would be a threat to him.
Obi-Wan to Luke in Return of the Jedi
Why would this be, unless Anakin’s children were expected to have something special about them that could threaten him? Certainly it wasn’t merely his flying or mechanical skills being passed on that would alarm the Emperor!
The Force runs strong in your family, pass on what you have learned.
Yoda to Luke in Return of the Jedi
This implies that Yoda is telling him to pass on this training to those he knows will have the ability to use it: the Skywalker family. If the Force has no physical component, then how is it passed on it families? By implication, genetics definitely has something to do with it. An adopted child isn’t going to be suddenly granted these Force skills just for claiming the name of a Jedi, be it Skywalker, Kenobi, Windu or anyone else.
The Prequel Trilogy


Midi-chlorians are a microcopic lifeform that reside within all
Qui-Gon explains midi-chlorians to Anakin in The Phantom Menace
living cells and communicates with the Force. – Qui-Gon
They live inside of me? ~ Anakin
In your cells. We are symbionts with the midi-chlorians. – Qui-Gon
Symbionts? ~ Anakin
Life forms living together for mutual advantage. Without the
midi-chlorians, life could not exist, and we would have no knowledge of the
Force. They continually speak to you, telling you the will of the Force. – Qui-Gon
They do?? ~ Anakin
When you learn to quiet your mind, you will hear them speaking to
you. – Qui-Gon
Tied together, the consensus of both trilogies is that a Jedi who does have children (whatever the Code may say) has a higher than average chance of producing a Force sensitive child. In fact, Revenge of the Sith connects the dots between the two trilogies. Anakin expresses pain that he has been kept out in the Outer Rim sieges, away from his wife. The Supreme Chancellor and future Emperor surely knew that keeping Anakin from his wife is one way to minimize the likelihood of her getting pregnant with a child that might complicate his already complex plans.
Expanded Universe Jedi Potential
When the Expanded Universe was fired up in the early ’90’s, the early comics had Jedi in families based on the original trilogy lore. This includes Nomi Sunrider and her daughter Vima, as well as Uliq and Cay Qel Droma, brothers. These stories are set four thousand years before the movies, which gives time for the Jedi to evolve into the ‘no attachment’ organization of the Prequels.
You have a device to test a potential Jedi student’s Force potential in Jedi Academy: Jedi Search by Kevin J. Anderson. It was apparently invented by the Empire in their quest to eliminate them. (This idea was requested by Lucasfilm.)
Natural Abilities
Even without a midi-chlorian count, there are indications of someone who has Jedi potential. Force sensitive individuals have the passive ability to see things before they happen. That doesn’t mean the person can manipulate the Force, they are only seeing more than others might. They also won’t know how they are doing it. Neither Luke nor Leia had a clue they had this power before they were told. In spite of his quick reflexes and dreams, Anakin didn’t either. But the Jedi (or Sith) may need to be looking for the signs in order to catch them.

He can see things before they happen. That’s why he appears to have such quick reflexes. It is a Jedi trait.
Qui-Gon to Shmi in The Phantom Menace
You mean it controls your actions? ~ Luke
Partially, but it also obeys your commands. ~ Obi-Wan
Luke and Obi-Wan discuss the Force in A New Hope
That ability to see things before they happen, or without actually seeing them with their eyes, enhances a person’s natural skills. That is, the skills they would have even if they weren’t a Jedi (for instance, flying, gambling, diplomacy.) This part doesn’t need training. It does need trust however, trust to allow it to control your actions. Someone who has never had training may have no idea they can rely on this instinct born of the Force. But it does need to be honed if a person is to become a Jedi. And to use the Force, manipulate it, that requires training.
Stretch out with your feelings. ~ Obi-Wan
Obi-Wan to Luke in A New Hope


We see in Attack of the Clones that Yoda early on is training the Jedi Younglings to ‘trust their feelings’ by using training lightsabers while wearing helmets that block their vision. Compare that to Luke, who started training late. While Luke did learn some of this from Kenobi, he had difficulty trusting the Force could be used in larger ways like lifting his X-Wing. This would be true of the majority of people who was untrained or familiar with the Jedi.
Training
Manipulation of the Force is the part that needs training. Telekinesis, levitation, combat skills, discernment and mind control, and many more, don’t just automatically come to anyone who is Force sensitive. For one thing (again) it would never occur to the average person to try and ‘envision’ that blaster leaping into their hand.
Young Anakin had more Midichlorians than any Jedi ever born. But that wouldn’t have enabled the boy to beat the likes of the trained Darth Maul. Even as an adult, when a fully trained as Jedi Anakin became Darth Vader but before he was crippled, he was unable to overcome Kenobi. Kenobi’s training was the leveler against Vader’s raw power. He had the strategy, as well as trained control of his own emotions in spite of the painful situation.

Like a muscle, a Jedi has to train to use the Force properly and strengthen their command of it. That involves all aspects, honing the natural ‘sensing’ abilities so they have discernment. Training the mind to have discipline and not let wayward emotions override that sense or skew its vision. Building up the body and movements for lightsaber combat, for even if you see that blaster bolt coming at you and enhance your blocking speed with the Force, the physical body can still fail you if its not prepared. The last thing you need is a muscle cramp in combat!
Even the most gifted Force User needs training to be a Jedi. In spite of their natural skills, both Anakin and Luke’s greatest feats of piloting came after receiving Jedi advice, however brief it was. Anakin had been in the podraces but never finished, much less won. Luke had flown skyhoppers, but the pilots shooting at the Death Star reactor before him had more experience, yet missed with a targeting computer. Luke succeeded by following Obi-Wan’s instruction.

“Feel, don’t think. Use your instincts.”
Qui-Gon to Anakin before the podrace, in The Phantom Menace

“Use the Force, Luke” “Let go Luke.”
Obi-Wan’s spirit to Luke during the Death Star battle in A New Hope
As well as the physical Force powers, there is the mental training. A Jedi needs more than the regular education in history, space travel, politics and typical life training. He also need the mental discipline and emotional control. What a Jedi puts into the Force can drive the battle. Hence, a Jedi Knight who is truly seeking peace, has to battle with calm. Otherwise they may drive the battle with their own anger and miss opportunities to turn things toward a peaceful resolution.
To tie the two trilogies together, we have Obi-Wan Kenobi. He blamed himself for failing to train Anakin to resist the Dark Side. This was a failure of discernment on the part of the whole Jedi Order. It wasn’t so much what Obi-Wan did wrong as what he failed to see. Palpatine (supreme chancellor and presumed friend) was training Anakin to be a Sith without anyone realizing it. Witness the difference, that Anakin gave in to his emotions whereas Luke, still attached to his friends, recognized he could not save them but turning to the Dark Side.
The culmination of the saga is when Anakin Skywalker turns back to his Jedi training and kills the Emperor. He witnessed his son succeed where he had failed. In that moment, when Luke’s life was threatened, all of his thought had turned away from himself. His act was selfless, like the Jedi he had once trained to be.


Old ideas, abandoned and not
While at one time George Lucas may have planned for anyone to be able to use the Force, the movies themselves show this is one (of many) things dropped in the drafting stages. Five out of six movies basically suggest it requires something just a bit special. Plenty of other things changed, from Anakin, Obi-Wan and Yoda returning in physical form, to Yoda being a wise guru but not an actual Knight. Nowhere, however, did it ever claim a person could become a Jedi without training.
References
- Midi-chlorians (original wiki post)
- The Complete Saga:
- The Phantom Menace
- Attack of the Clones
- Revenge of the Sith
- A New Hope
- The Empire Strikes Back
- Return of the Jedi
- Jedi Academy: Jedi Search
- Tales of the Jedi: The Collection (Uliq Qel Droma & the Beast Wars of Onderon, The Saga of Nomi Sunrider)
- Tales of the Jedi: The Freedon Nadd Uprising
- Tales of the Jedi: Redemption
