The creed consists of three principles:

1. "Do not kill innocents."

2. "Hide in plain sight and be part of the crowd."

3. "Never endanger a brotherly alliance."

Regarding the understanding of the Assassin's Creed itself, the following passage from Assassin's Creed 2 is also mentioned in the game, and was mentioned by  Ezio when he joined the Brotherhood:
1:  These are the words spoken by our ancestors that lay at the heart of our creed.

2:  Where other men blindly follow the truth, remember--Nothing is true

3:  When other men are limited by of laws, remember--Every thing is permitted.
permitted.

4:  We work in the dark and serve the lights, we are assassins

5： Nothing is true, everything is permitted.

The other part, from Assassin's Creed: The Revolution, is Arnold's own insight, and I think it's a good one, too. The Brotherhood of Assassins' dogma teaches us that all things are permitted, and I used to think that meant we were free to do what we wanted to do, and to pursue our ideals no matter what the cost. Now I realise that not everything is permitted, but that the dogma itself is a warning. Ideals are too easily compromised with doctrine, and dogma makes men fanatical. There is no right higher than our own judgement. Nor is there a Supreme Master watching who can punish us for our sins. At the end of the day, only we can avoid over-obsession, and only we can decide if the way forward is too high a price to pay. We always think we are the redeemer, the avenger, or the saviour. We wage war on those who oppose us, and then they in turn wage war on us. We always dream of leaving our mark on the world ...... but we only sacrifice our lives in vain in wars that the history books will not record. What we have done, what we have now, will fade away with ourselves.