How can you be sure that God is outside of your mind instead of inside? Every spiritual or religious person seems to have their ideas about what God wants them to do, or what he (or she) is like. I am not a Christian, but very interested in the idea of faith. I am trying to decide whether it is a good thing or not in society.
Yours is a question that many believers struggle with as well. I sometimes have this thought come into my mind, even after many years as a Christian. I think this question comes down to asking whether God is real–does God exist?
I am afraid I cannot give you "proof" of the existence of God such as what you will find in a geometry class. Here is the question I would like to challenge you to consider: What is the most reasonable conclusion with regard to the evidence both for and against the existence of God? I believe that if you approach this question with an open, unconditioned mine, you will find that it is vastly more reasonable to believe that God exists than to conclude that he does not. Atheism is not irrational, but it is not the most rational view. Atheism does not work, either as a practical philosophy for life, or as an explanation of reality.
I have done a presentation of the existence of God in a number of countries. There is an outline and power point for the presentation at the web site. Here is the URL: https://www.evidenceforchristianity.org/index.php?option=com_custom_content&task=view&id=5241&&Itemid=52
You may find this helpful. To me the strongest arguments for the existence of God include:
The Moral Argument
The Cosmological Argument
The Teleological Argument
Argument from evidence for the inspiration of the Bible
Argument for the resurrection of Jesus
I find the arguments that God does not exist to be very weak. Of course, you will have to decide for yourself.
Also, I have written a book on the subject. It is "Is There a God?" It is available at www.ipibooks.com
Lastly, I want to ask you to consider a challenge which Jesus gave to those who were considering his teaching. "If anyone chooses to do God’s will, he will find out whether my teachign comes from God or whether I speak on my own." In other words, Jesus is challenging you to consider giving faith/obedience to him a chance. He predicts that if you choose to obey him, you will have experiential evidence sufficient to convince you that he is from God. I hope you will be blessed in your attempts to think about this fundamental question. Feel free to ask more questions if you like.