The Knights Templar was a mysterious order of fighting monks. They were founded in 1099 and for the first nine years counted only nine members. In those years they were digging under the old temple of solomon.
They were obviously searching for something. After nine years a kinght was digging,so the story goes, at exactly at noon one day and saw in the light of the sunray comming in from above, a vere low opening in the wall of the level were they were digging. An opening, so low to the ground, that it was not visible when you were standing up. Except when the the sun was shining precisely from above. Whatever they found, it is the secret of the Templars. A secret so powerfull, that people are willing to die, or to kill for it.
Together they went to Rome and consulted with the pope what to do next. From this we can asume that their secret find had something to do with the Roman Catholic Church.
We know now trough the Nag- Hammadi scrolls that jesus didn't die on the cross, that Mary Magdalena was his wife (they had a child), and that there was an on-going dispute between Mary and Peter.
Whatever they found, it is said that the pope paid his respects to it, but it was not for the masses to be known.
You can imagine that proof of Jezus' mortality, would not be welcomed by the church. Nor proof that he was maried to Mary, or any proof of a child.
Anyway, it has to be something that compromises the church, otherwise there wouldn't be any reason for it to remain a secret.