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