It has been said that Assassin's Creed is half reality and half fiction. Which means that Templars are completely real, but it is the Assassins that are fictional.
La shay'a waqa'on motlaq bal kollon momken
If nothing is true and everything is permitted, then it is not true that nothing is true. But truely nothing is true, that is why it's true. Though nothing is true, we should hold these nontrue words as our guide to transcend. Because it's not even supposed to be true.
To recognise that nothing is true, is a way of seeing through the illusions.
But what does "nothing is true" has to do with "everything is permitted" ? Although we may recognise that nothing is true, we cannot live without illusions, because human life is based upon them.
We all have our own illusions that are different from one another. To recognise that everything is permitted is a way of breaking through the illusions. It makes us remember that anything can be our illusion, and it is we, who should choose what kind of illusion must be ours.
Friedrich Wilhelm Nietzsche says in his famous book "On the Genealogy of Morality":
"When the Christian crusaders in the Orient came across that invincible order of Assassins – that order of free spirits par excellence whose lowest order received, through some channel or other, a hint about that symbol and spell reserved for the uppermost echelons alone, as their secret: nothing is true, everything is permitted. Now that was freedom of the spirit, with that, belief in truth itself was renounced."