I think it might be the original doctrine of the Jedi Order, the basic founding principals upon which the Order was founded that the Order abandoned or misinterpreted after thousands of years. Like what the EU Jedi Order became as opposed to the Je'daii. I think Luke searched out the first Jedi Temple in an effort to find this text in his goal to learn whether or not he should disband the Jedi Order or attempt again to rebuild it.