Still, I'm wondering that I couldn't find an answer to his answer- that the belief in God is an intrinsic part of him
I think that the belief in god can be a component of self-identity, and also a way for the human mind to relate to the experience of the inner sense of self, the sense of "I-ness". So at his current level of development, he has an inner relationship with his sense of "I-ness" in which he needs to have a god to make sense of things, and therefore his religion feels like an intrinsic part of himself. It is good for those in Epicurean philosophy to understand when they see this in others. Eventually people grow out of their need for myths. (Also see Principle Doctrine 12).
This will shed more light:
Ep. 6: Joseph Campbell and the Power of Myth -- 'Masks of Eternity' | BillMoyers.com
Bill Moyers and Joseph Campbell discuss the common experience of God across cultures.
billmoyers.com