I have no problem saying that God exists in the human mind. However, that is completely different than any kind of God actually existing. I want to use prayer as an example to start. Prayer works completely free of any divine intervention, it is psychological, the power of the mind. Let us say that a religious believe is in the hospital. This person strongly believes in a loving and caring God who is watching out for them and interacts in their lives. Meanwhile, they have a huge congregation of people who believe the same, they all believe that there is a caring God and asking Him to help works. So, we have a person who starts getting better when he is well aware he is being prayed for. Is there anything supernatural about this? No. What is happening is simply the power of belief and of the mind helps him heal himself.
We can further show this. Praying for someone with pneumonia can help greatly, many religious people have seen prayer work and accept it as divine intervention. Why then, do no true miracles ever happen? If a person survives pneumonia with the help of prayer, that may be accepted as divine intervention. However, if the same person loses an arm, no matter how hard you pray it will not grow back. It seems evident that no great feats can be accomplished through acts such as prayer, as there is nothing involved but the human mind. It is completely natural for a person to recover from pneumonia, and positive thinking alone is proven to work. It is not natural for limbs to grow back.
So what is it? Is prayer the power of the mind, or are religious folks simply not praying for anything that the mind and modern medicine cannot fix? Another example can be starvation. It is not the fault of man that people are starving all around the world, and even if you could argue that it is, God should have the ability to feed them and not punish them for the evil nature of others. Yet, the hungry stay hungry. So, why are the religious not praying for them? Well, they are, but the power of the mind cannot feed entire countries.
What it comes down to is either that God is simply in the mind, or he has very screwed up priorities. I mean, if God will get rid of your flu but not help feed the hungry, is that really an entity worth honoring?