It's relatively easy to conclude that God exists by self-reflection, and attention to consciousness. First notice the mystery of being 'alive' at all. Because we only return to ourselves when we relate to the eternal, so it is clear that this eternity does exist.
God has given us his mind, perhaps. We have been given this eternal Energy (to speak with Gregory Palamas), which is reflected in our uneasy shattering existence. It is easy to conceive of God because, without this eternal Energy flowing in us and around us, (which it is necessary to acknowledge in order to benefit from), being alive is kind of - pointless and utterly disorganised, painful. I have never been at home in existence, and my mind belongs, in part, elsewhere. And recognising God is simply the upshot of this difficulty which the individual has when living - at all times.
Silent prayer, leading to the depths, makes the belief in this force or great Mind easy: somehow we go home when we meditate. We exceed ourselves and our private individuality, and enter into company with that other person who was there always with us.
It is senseless to debate the existence of God; even among unbelievers it is easy to confess the absolute certainty we have in His existence. And when I say 'we' I mean those of us who have learned to commune with God in prayer.