In the beginning God, then God was not, for God became the Cosmos.This is pandeism in a nutshell.
We might have a variety of intellectual reasons to consider the concept of God as necessary, perhaps as a first cause, or the being that 'fine tuned' the universe for life and consciousness. However God is apparently absent from our Cosmos today. Not least we have the problem of evil, which demands that God intervene if he omnipotent and good, yet he does not. The claims of religious persons to have encountered God or to have received a revelation are not verified, incoherent or simply conflicting. Pandeism explains why we do not find a supernatural 'person' called God present in our world. Pandeism does though explain the widespread intuition of many people that nature is in some sense sacred and that there is deep mystery grounding our existence.
Pandeism has I think, this intellectual difficulty. If God became the Cosmos via an initial singularity event, then any God nature would appear to be obliterated by the Big Bang. Nothing that seems to be specifically personal can be carried over into the new Cosmos where this has begun as result of a point of infinite temperature/pressure etc. God does not so much become the Cosmos, but dies and is cremated and the result of this death is that the residue of this God-destructive event becomes the Cosmos. So in a scientifically consistent pandeism the Cosmos is not 'God' in any meaningful sense, any more than my cremated ashes are 'me' even if those ashes were to be incorporated into some other structure or life form. So Pandeism seems to require, in light of scientific knowledge of the initial singularity, that God is now dead.Thus in fact it becomes a strange form of atheism-theism hybrid: there was a God, but not any more.
With my knowledge of process theology (that of Alfred Whitehead and Charles Hartstone), I would borrow some concepts from that theology to substantially modify pandeism. In my version God does not 'die', but God exists in a hyper dimensional reality that is not our reality. God does not exist in our 'space-time' as an absolute individual being and cannot intervene as an 'actor' in space-time (so this is still a deism). God may still be 'conscious' of all that goes on in our universe by virtue of existing hyper dimensionally. However space-time is a process, an unfolding of the deity's 'consequent' nature, which seeks to evolve ever greater consciousness and creativity in physical form. God's relationship to the universe is rather like that of mind-body, except this mind can only set up the program for the body, it cannot 'move stuff' around. You might say 'well of course I can move my body' with my mind. But what I'm talking about is not the manipulation of our environment via our limbs, fingers etc, but manipulating our internal physical constitution. I can't for instance will my blood vessels to move differently - our bodily processes are generally autonomic. But anyway God has no 'environment' if God is infinitely and omnipresent. Our universe's relationship to God is analagous to the relationship not of limbs or fingers but of the cells of our bone, skin and blood to our mind. We can perceive important physical feelings but not direct their functioning. Our body is a 'given' and we can't chose for it to be anyway other than it is.God's body, the cosmos, is designed by God, but its subsequent evolution is not entirely determined or predestined (quantum mechanics requires that complete determinism of the future is not possible). This is where the human mind-body analogy breaks down. God would know all possible futures and would probably set an ultimate 'course' for the body to follow, though beings have some freedom within these initial parameters set by God.