Sometimes I discuss spiritual matters with non-believers, and sometimes with ardent non-believers who do not just dismiss religion and spirituality but see them as positively pernicious, the product of superstition and ignorance. Often on these latter occasions I end up being accused of ad hominem arguments in which I supposedly play the man not the ball. I have to concede this is very probably true but that is because of my sense when I speak to a person like this that they believe what they believe, or refuse to believe what they want not to believe, because of the sort of person they are.
The root of faith is in the will not the mind. There is plenty of evidence for God in scripture, nature, the human mind and life itself, but there is no overwhelming intellectual evidence nor meant to be since that would remove the essential role of free will in the matter. This means whether we believe or not depends on the sort of person we are, on a natural tending towards God or one away from him. God is truth and love so this means that whether we believe or not is ultimately down to whether we respond to, value and treasure truth and love, particularly whether we do so above ourselves Doubt is permitted, that is a normal part of the human experience, but outright rejection is not. And even doubt should not be a permanent or dominant state.
I will not go so far as to say that good people believe in God and bad people do not but, in spiritual terms, there is certainly an element of truth in this. If you accept God that means your heart is open to the spiritual heart of the universe, and this essentially is goodness. If you reject God that means your heart is closed and that essentially is evil. You may be a moral and virtuous person in your everyday dealings with other people, but that means very little if you refuse to acknowledge your Creator. It implies there is an egotism and a self-will within you that denies and rejects the source of all truth and beauty and goodness. In this sense, you are what you think.
Critics of this point of view will point to the many people who claim they believe in God but behave badly. I would say two things to this. Firstly, we are all sinners and fail many times. What counts is whether we repent our failures and sincerely try again. Secondly, there are, of course, many hypocrites and self-deceivers who use God for their own ends. These people clearly do not believe in God. If you believe in God, you love him and keep his commandments or, at least, try to keep them. There is belief of the head and belief of the heart. I am talking about the latter here which is the only one that counts.