How can we know "God" exists?
By surrendering yourself to all that is. By acting in unison with the wisdom that flows through you and is within everything. By discerning, in all things, between the real and illusory, the Absolute and the relative, the changeless and the unchanging. By never thinking about yourself. By not flattering your vanity. By not aiming, therefore, for originality of thought, but for True thought - originality will come of its own. By not trying to possess or claim credit for truth. By opening your ears to the voice of intuition, and ignoring your own babbling. By not getting lost in distractions like this discussion. By surrounding yourself with beauty. By loving your neighbour as yourself. By giving everything of yourself, until you are nothing. In short, by following a traditional religious path. Without religion, you will discern, if anything, only faint glimpses of self-transcendence (or pseudo-transcendence) in fleeting moments of self-forgetfulnss, as when contemplating a work of art, or enraptured by the beauties of nature, or entranced by a glimmer of that Edenic timelessness that some associate with déjà vu: but inevitably you return to the vanities and the illusions of this world, a prisoner of your own mind.
While God is beyond labels, symbols and ideas, one must be oriented towards God in order to see God; consequently, one must have correct doctrine, rooted in religious orthodoxy, and assimilated into oneself by correct practice. Correct practice builds and maintains the seaworthy vessel, and correct doctrine ensures sound navigation. Lacking these prerequisites, no man will fortuitously experience God (except in relatively uncommon cases as when god seems to take the initiative and reveals himself to the suffering individual who has no means of escape, but in these cases there is an antecedent period of intense pain in which the soul opens itself to God; religion achieves the same end by doctrine and practice).