As far as I can understand it, it is irrational to believe in the Christian God. 2 Corinthians 5:7 declares that "we walk by faith, not by sight." This is quite nearly an admittance to blindness. Faith is unjustified belief.
In my experience, I have found not a single reason to believe in the Christian, or any other, God, and that the Bible is nothing more than a fairy tale.
There is no "true" Christianity, as is obvious by the presence of a multitude of different Biblical sects, each of which interprets the Bible in a certain way. Which one is actually truly right? Neither of them are, because in each case, believers simply adhere to an interpretation (their own personal one; I am not referring to translations) of the text. When the text is colloquial, archaic, non-technical, and even in some cases metaphorical and prone to allegory, there is no one true meaning. If there is, you are quite welcome to try to prove that yours is the correct one.
It's impossible to prove a negative unless a valid positive argument has been presented. However, the second clause implies that a valid positive argument must be presented before a negative is to be even attempted. Since there has not been a single sound argument for the existence of a Judeochristian deity, there is no reason at all to believe in it, and one may as well disregard the possibility of its existence. A good example is six-foot long invisible snakes which like to live under beds. You certainly have no proof that they don't exist, but you don't honestly consider the possibility every time you get out of bed.