Q: Do you believe in God? A: What do you mean by "God"?

If somebody asks: Do you believe in God? 

Then the true answer depends on what is meant by "God" - and there are very wide divergences indeed between understandings what is meant by "God" - even among Christians. 

The only valid answer would be some version of: First; what do you mean by God? 

So, you cannot, and indeed should not, answer the question without getting clarification of what the enquirer is assuming about (what he is calling) God.

Otherwise, by saying "yes", you are assenting to buy a Pig in a Poke. 

For example, my understanding of God, the God in whom I believe;  is one that many/most Christians, Jews, Muslims would not consider to be actual God at all - or, at best, only a selective sliver of what they believe God to be. 

To such people, because I regard the Omni-God as false (and indeed incoherent), I am a kind of pagan - whatever my attitudes to Jesus Christ, whatever my desire for salvation.  

While to me; the Omni-God believers (insofar as they are real Omni-God believers, rather than those who are merely parroting forms of words, in obedience to the authority and doctrines of their church); are all de facto monotheists...

Monotheists whatever their Trinitarian protestations; and as such they do not really regard Jesus Christ as essential to anything (not even salvation) - nor do they believe in evil or freedom.   

And I regard all Omni-God monotheists as metaphysically indistinguishable from proponents of Oneness spirituality - and therefore their many practical differences are varieties of incoherent with their metaphysics.

I mean that the Omni-God is reducible-to/ metaphysically-indistinguishable-from the "deity" of "Deists" - which is not a person, but Just Is. Such a God-Deity is The Way Things Are.  

Answering "yes" to "do you believe in God?" therefore means very little or nothing - and a "yes" is more like to mislead than enlighten. 

Of course, in the West hardly anybody believes in God of any kind - so a negative reply to "do you believe in God?" is usual, and informative.   

But before assenting to belief in God we ought to be clear what we really mean by this - what we regard as vital to our understanding of God. 

Once someone has honestly (and without veiling-abstractions that serve to disguise incoherence or incomprehension) explained what he means by "God" - he has gone a long way towards explaining how he regards ultimate reality. 

He has also, potentially, done himself a big favour - because it may be that, once he has explained what he means by God, in terms that he himself can clearly grasp - then he will discover that this God is not really what he does believe or desire: in his heart of hearts.