I don't understand what you're asking? I didn't say that John existed before Reese went back in time, I'm saying that in order for John to exist, Reese had to go back in time. So those three things had to have happened in order for there to be a John Connor. Everything else after the first Terminator movie could be scrapped if the writers of Salvation were so inclined.
Wasn't directed toward you specifically, just a general question that's confused the piss out of me for years now. How could John send back Reese to father John? I mean, John had to exist at some point from some other father, or how else could John be there to send Reese back originally? Doesn't make any sense to me.
Reese only had to go back in time to make sure Sarah Conner survived. A "John Conner" had to exist in the future. The machines knew that John Conner was the leader of the resistance and that his mother was named Sarah Conner. The resistance was about to win the war and in a despirate attempt to destroy the resistance, Skynet sent a Terminator back in time to kill Sarah Conner before John Conner was born. Therefore, in the original timeline Sarah Conner had a kid with someone (not Reese) and named that child John. That John Conner survives Judgement Day and ends up in a work camp, gets freed, and lead the resistance. He then sends Reese into the past to stop the Terminator as seen in T-1. Thus, history is changed because Reese now becomes the father of John Conner. Since he knows that Sarah is the mother of the leader of the resistance, he tells her everything and thus influences the future.
Now we have a new timeline, where Sarah can prepare John for the future that he will face. At some point, Sarah is commited to an asylum and John grows up in a foster home. Dyson finds the Terminator remains and now he creates Skynet earlier. Judgement Day occurs earlier and by the time the resistance wins, Skynet has greater technology. They send back the T-1000 to 1994 to kill John Conner this time as seen in T-2. John Conner has also figured out how to reprogram Terminators and sends back Arnold. The result is that Judgement Day is now pushed further into the future, but it still happens as seen in T-3. SCC jumps the T-3 timeline to put Sarah past hear normal death date and thus creates another alternate timeline.
Essentially, a "John Conner", son of Sarah Conner, had to exist in order for T-1 to take place. I can be debateable as to whether the John Conner who sent back Reese is the same as the one that Sarah is pregnant with at the end of T-1. Myself, I would say it is impossible since Reese would never have gone back in time without John sending him.