It is reasonable to deny the existence of God, if by 'God' you mean some being external to yourself who hangs around in the sky either interfering or not interfering in the creation (since either causes problems). Also, a god who is supposed to be the creator of everything begs a number of questions, such as 'who created God?'
If however we begin to look at the world from the bottom up, so to speak, we realise that the whole thing is a living mystery. Just being conscious is inexplicable. We can talk indefinitely about neural networks and emergent properties, and still not understand how I, now, can be conscious, or even why the consciousness that contains me seems not to be the same as the consciousness that contains you (why am I me and not you?). (This is a question that has perplexed me since I was five years old and my toy rabbit was confiscated in school and put on a shelf where I could see it and not get it. I remember thinking, why am I me and not that toy rabbit? Of course I now understand that cloth animals are not made in such a way that they can be conscious, at least it is reasonable to suppose so, but the question 'why me here now?' remains.)
The question becomes less or more confusing (depending on our point of view) if we start to look at it from a god-like perspective, as it were from the outside, as if we could see the whole creation, and imagine creating beings such as ourselves wired up in such a way that we can exhibit complex behaviours. Then we can say, well, that one thinks it's conscious because that's how it's made, and so does that one and those over there. No mystery really. But actually we are only privy to a god-like perspective in our imaginations - that's not how it is.
Then again, if God exists it follows that God is more than our human understanding, and therefore we should not expect to be able to reason about God, except to say that logically it follows that we cannot reason about God.
Why have what the atheists would call an unnecessary hypothesis? Let us go with Hippocrates and ask for no unnecessary hypothesis. Then if we glimpse something more than the imaginary world in which we say we live we can allow space for what we don't know.
From Robert Graves's poem Warning to Children:
"Children, if you dare to think
Of the greatness, rareness, muchness
Fewness of this precious only
Endless world in which you say
You live, you think of things like this: ..."