What specific event was the forking point between the two storylines?

Question

In Rebuild of Evangelion, the first movie is almost an exact retelling of the original anime series. But soon it begins to deviate more and more (and more /sad...), until I really wasn't sure of what I was watching anymore (but I digress).

So, lets consider both storylines as happening on parallel universes.

What is the event (onscreen or not) in the Rebirth series that is the turning point of the forking/deviating storylines?

---

While this may seem opinion-based, it is not. I am asking you to compare the rebirth movies to the original series, script or storyline-wise, and backtrack the visible differences to their sources, pointing to a specific event that happened onscreen or was mentioned that may be the cause of those differences, looking backwards for the cause of that other event. Pepper your answer with references, please.

You may add a few speculations in making your chain of causality, if the events have a significant impact or relation.

Answer 1

Here's a list of differences between the first movie and the original timeline

The most notable plot-wise is that Misato knew about Lilith, and she knew what Lilith's purpose was, and later before the Ramiel battle, she showed Shinji what Lilith was and explained why he needed to fight the Angels.

The appearance and identification of the Angel Lilith at the end of the first movie and just before the commencement of Operation Yashima (Corresponding to Episode 06 of the TV series), unlike the veil of secrecy kept in the original series where Lilith was never seen until Episode 15 and wrongly identified as Adam until Episode 24.

More notably, Gendo and Fuyutsuki speak about a "contract with Lilith," which did not exist in the old series. Lilith's existence may not be a mystery, but its role is hinted to take a different turn from its original series counterpart.

There was no mention of a "contract" with Lilith in the original time-line.

Answer 2

I've only seen up to the second movie. However, if i was to make a guess, it would be what Kaji gave to Gendo.

In the original one, it was a Adam's Embryo, while in Rebuild it's Nebuchadnezzar's Key. I'd say its discovery (before the first movie) is what would be turning point, but a noticeable difference didn't occur until Kaji went to deliver it to Gendo.

In the original series, Gaghiel attacked the UN Pacific Fleet searching for Adam's embryo, but that never happened in Rebuild. Rather, Ramiel became the new 6th Angel, while Gaghiel never appears. Instead, we have the unnamed Third Angel which, if you account for the fact it was subjected to experiments, is reminiscent of Gaghiel.

Also, Kaji released the Third Angel to cover his escape with Nebuchadnezzar's Key, so it's possible that the facility that the Third Angel was in was where Gaghiel attacked before the first movie. Gaghiel may have been captured and experimented on, and became the Third Angel we see in the second movie (thus making Sachiel the Fourth Angel). Kaji knew that by releasing the Third Angel, the facility will do everything they can to prevent its escape from the facility, and in the end, they destroy it.

I'd also note that Sachiel, Shamshel and Ramiel are very much the same since the first movie was more of less the same as the original (very little deviation), except their numbers have been pushed up thanks to the Third Angel appearing in the second movie (and numbering seems to be based off appearance) - from the Seventh Angel on they are different.

While Bardiel is just demoted in number, Asuka is the one who pilots the Eva, not Toji, and the Dummy Plug is over top Shinji and talks backwards which didn't happen in the original.

Jon Lin's Answer talks about Lilith which I totally forgot about. However, both Rebuild's references to Lilith in the first movie and the original series references to her would still occur after the item Kaji gave to Gendo was discovered (Original = "Adam's Embryo", Rebuild = "Nebuchadnezzar's Key"), and as such I stand by my speculations.

Answer 3

I think one of the biggest changing points is the Vatican Treaty.

As the wiki points, this represents a weakened political influence by the part of Seele. Comparing the roles of Seele in both continuities, in NGE they are the masterminds, but in Rebuild they are more like an advisory board (going even to the extreme of shutting themselves down peacefully, instead of having the plug pulled by Gendo).

The Vatican Treaty may have arised from a stronger nationalism to fill the vacuum left by Seele's reduced influence. Since Asuka identifies more as a German (even being part Japanese), this reduced the bonding between her and Shinji and probably supressed whatever crush she had on Kaji.

But keeping with the Rebuild continuity, there were no need to a fourth pilot, so Toji Suzuhara was never recruited.

When Bardiel attacked, the one inside the plug was Asuka, not Toji. The bond between Shinji and the person trapped was smaller, and the psychological damage from the fight was also smaller. Toji was Shinji's friend as much as in NGE, but Asuka was, instead of a love interest, just some annoying female.

This vacuum in Shinji's love interest created by Asuka disinterest had him gravitate more towards Rei. So the psychological reaction to having Rei "dying" while fighting Zeruel had the added impact that was missing during the fight with Bardiel.

So instead of just going into 400% synchro like in NGE, Shinji causes the awakening of EVA-01. Notice that in both scenarios, EVA-01 ends up bearing both fruits (in NGE she eats Zeruel S2 engine, while in Rebuild the liquified Zeruel-Rei merges with EVA-01), becoming the forbidden godlike being.

So I think the root cause (as far as we can go from what is shown in the Movies) might have been the Vatican Treaty.