The only place we don't really know everything about the characters before their story begins is in the Marvel X-Men titles. The basic rule is obscurity. It was how it was when the series first started, with Professor X turning his loyal students out to battle Magneto without anyone really knowing any of their backstories. This continued on, with characters turning up and hints but no answers, such as Bishop recognizing Gambit from the future. Or even just the massive "Wait...what?" that is the origin of Wolverine.
Into this morass of mystery came one of the most polarizing creations of the day, Cable. Bob Harras, the Marvel mutant editor at the time, felt New Mutants needed to get shaken up a bit and asked Louise Simonson, the current writer, and Rob Liefeld, the current artist, to gin something up. Simonson figured the result would be a more military leader, and started coming up with storylines involving "Commander X."
It is important to note that Liefeld claims the name "Commander X" was lame (like "Cable" is some sort of improvement), but Simonson denies she intended that to be the name -- it was just a placeholder for what she was writing, and she'd insert the "official" name once the final decision was made. This might seem like a silly point to bring up, but it does tie in with where this eventually goes.
Harras and Liefeld were buddies, and Liefeld was encouraged to do whatever to create this new character. They were likewise thinking military leader. Liefeld wanted the opposite of Charles Xavier (which we already had in the form of Magneto. But I digress). Liefeld's only real instruction from Harras was that "a bionic eye might look cool." Liefeld came up with a slew of sketches and backstories. Harras eventually settled on a mysterious time traveler that we now know as "Cable."
The first appearance of Cable was also the first appearance of his enemy, Stryfe. Liefeld kept details of the two characters intentionally vague. In fact, the time-travel aspect wasn't revealed until several issues later. He wanted a Wolverine-type thing -- Wolvie's history was constantly being tinkered with and new revelations were constantly popping up, and he wanted that kind of thing with Cable, and with Stryfe tied so closely to Cable, that meant he had to be kept vague as well.
The fandom pretty much split down the middle. There were people who thought Cable is a pretty cool guy. Eh kills mutants and doesn't afraid of anything. Then there were people like me who saw Cable as the anti-thesis of Xavier's dream and wanted Magneto to kick his ass and show him how it's done. Liefeld continued to write his stories, content with one important fact. He generated the character in a bubble. Harras gave him no instructions and didn't ask too many questions. Simonson wasn't involved in the creation process of the character. No one else in the creative team knew anything about the history of Stryfe and his connection to Cable.
In other words, there was only one person in the entire world who knew exactly who Cable really was.
Fan speculation ran rampant. Some thought Cable was actually Nathan Summers. Another theory was that he was Ahab. Some were speculating that Stryfe was actually a clone of Cable, others felt the opposite (clones were big at the time). Liefeld's original idea for who Stryfe was was actually pretty interesting -- Stryfe and Cable weren't clones of each other, they were the same person. Stryfe is a future version of Cable. Cable fell back in time and memory holes erased part of his memory, including who he really was. Liefeld figured that we didn't know exactly where in Cable's history we were when he turned up in the present day, and originally intended to follow Cable's descent into madness and eventual turning into Stryfe (see the last page of New Mutants #100 when Stryfe takes off his helmet for the first time). I talk a lot of shit about Liefeld, but credit where it's due, that's actually pretty interesting.
Things got dicey when Whilce Portacio, Jim Lee, and Bob Harras brainstormed during preparations for the X-Ecutioner's Song crossover. They decided that Cable was actually Nathan Summers from the future and Stryfe was a clone of Cable. Realizing his grand plan and revelation was scuppered, Liefeld kept his mouth shut. As debate over who created Cable continued, one of Liefeld's comrades at Image accidentally stoked the flames as to his true identity. Jim Valentino wrote to Comic Buyer's Guide that he had had enough and, taking Liefeld aside, asked him point blank who Cable really was. Liefeld made him promise not to tell. Valentino agreed.
Liefeld told him.
Valentino wrote that he never expected who it was in a million years. He could now see the hints of what would happen, but he never put it together.
But Valentino never revealed who Liefeld told him it was.
And the speculation continued. People asking, people wondering. The only real clue was that Liefeld had stated he never intended Cable to be Nathan Summers. What about Ahab? Ahab was the focus of the mutant books' crossover that took place in the annuals. But Claremont wrote the storyline, and he wasn't involved in the creation of Cable. Fabian Nicieza has stated on the record that Harras suggested putting the hints in there as a red herring to throw off readers and keep them guessing. So Cable was not intended to be Ahab.
So, if Cable was not originally going to be one of those two, who could he be?
Liefeld was asked by Brian Cronin of Comics Should Be Good about a rumor that had been circulating. The rumor stated that Cable was actually a future version of Sam Guthrie, a.k.a. Cannonball.
Liefeld told him yes.
Liefeld's idea was that the memory bubbles Cable got subjected to when he fell through time erased his knowledge of his identity. So he had no idea was actually mentoring himself when he took over the New Mutants and formed X-Force.
So there you have it, straight from the horse's mouth.