Don’t even know who the chap is, but don’t need to, it’s pretty obvious.
This is a parody of the Islamic profession of faith, There is no God but Allah, but Muhammad is His Prophet. As His Prophet, Muhammad systematised and presented to the community the understanding of the divine that is Islam. Muhammad conveyed from Allah an understanding of the divine, through the Quran.
Mccarthy’s statement is:
- There is no God [at all]: i.e., Atheism
- We are his prophets: instead of Muhammad, each of us constructs our own understanding of the divine (acts as a Prophet), because that understanding is ultimately a response to human concerns, and not a revelation from beyond.