Prepositions can be very eloquent. A man is "in" architecture or a woman is "in" teaching, we say, meaning that is what they do weekdays and how they make enough money to enjoy themselves the rest of the time. But if we say they are "into" these things, that is another story. "Into" means something more like total immersion. They live and breathe what they do. They take it home with them nights. They can't get enough of it. To be "into" books means that just the sight of a signed first edition of Alice's Adventures in Wonderland sets your heart pounding. To be "in" books means selling them at B. Dalton's.
Along similar lines, New Testament Greek speaks of believing "into" rather than believing "in." In English we can perhaps convey the distinction best by using either "in" or no preposition at all.
Believing in God is an intellectual position. It need have no more effect on your life than believing in Freud's method of interpreting dreams or the theory that Sir Francis Bacon wrote Romeo and Juliet.
Believing God is something else again. It is less a position than a journey, less a realization than a relationship. It doesn't leave you cold like believing the world is round. It stirs your blood like believing the world is a miracle. It affects who you are and what you do with your life like believing your house is on fire or somebody loves you.
We believe in God when for one reason or another we choose to do so. We believe God when somehow we run into God in a way that by and large leaves us no choice to do otherwise.
When Jesus says that whoever believes "into" him shall never die, he does not mean that to be willing to sign your name to the Nicene Creed guarantees eternal life. Eternal life is not the result of believing in. It is the experience of believing.