In the Mormon Church I still hear many people testify that they "know" God lives or that they "know" Jesus is the Savior, etc. Some Mormons and other Christians also claim that God has "proven" his existence to them or that they know God exists because they have a personal relationship with him. Other Christians instead wisely say that they "believe" God exists or have faith that God exists.
There is a large distinction between knowing and not knowing, yet believing. It is my position that one cannot "know" that God exists and anyone who believes they know are likely forgetting or ignoring the assumptions and subjective interpretations that they are making about their experiences.
Now, these people who claim to know will often confess that they cannot prove God's existence to you, but that you must go to God to get your own proof as they have done. They often believe that we atheists have never had the kind of experiences they have had, but that if we did we would believe too. Or, sometimes they think we are too rash in dismissing personal experiences, since their evidence is not of the scientific kind. Allow me to clear up a few things. One, I have had the types of experiences that "knowers" and believers have had. Two, I accept those experiences as evidence, not proof, but I limit my acceptance to the raw experience only, not to the meaning that has been given to that experience. Let me give a few examples.
A woman claims that God spoke to her during her prayer last night. Upon further questioning it is discovered that she did not actually hear a voice, but instead had a thought that gave her peace and offered a solution to her problem that she believes could not have come from herself. I challenge the position that it was impossible for her to come up with the idea by herself. But, she just says she knows it was from God as an answer to her prayer.
What was the basic phenomenological experience? She had a thought accompanied by a feeling of peace while speaking about a problem. That is what I accept as evidence. All of the rest is conjecture, assumptions, and interpretations. One does not "know" that thought came from God. One did not experience God directly, all one did was have a thought accompanied by peace while speaking to an unseen being that might be real and might not. Might this be thought of and used as evidence for God's existence? Certainly. Might this be thought of and used as evidence for the natural power of the brain? Certainly. The experience itself proves nothing. It is not impossible that the thought and feeling of peace was entirely a creation of the brain. To any who claim that it is impossible, I ask "impossible? Impossible?" Surely it is possible, why would it be impossible? Regardless of the different probabilities we might assign to the possible explanations of the event, we both must admit that either interpretation is possible.
The existence of the earth and the variety of life upon it is often claimed to prove that there is a Creator. I will admit that if God did create it, it is evidence of his existence. But, is it not also possible that the universe and all that is in it came about by natural (uncreated) laws without the involvement of a Creator? It most certainly is possible. If you disagree, shall we review the law of large numbers, gravity, and evolution by natural selection? You may not think it likely, but logic demands you admit that it is possible even if you refuse to imagine it.
Look, all the "knower" has are raw experiences and events. Do not accept your interpretations, and assumptions as givens. For they are debatable and they are not part of the raw experience. You do not "know" that God changed your heart. You do not "know" that God answered your prayer. You may know that you experienced a change of heart, but you do not know that it came from God. That is but one interpretation that is often accepted as part of the experience, but it is an assumption that might be incorrect. If you have difficulty imagining or buying the possibility of a naturalistic explanation for a phenomenon (near-death experiences, prophecies, etc), ask a naturalist and they will explain until you get it.
"Knowers" have no proof. To prove something, you must be able to demonstrate that there is no possibility that any alternative explanation could adequately account for the phenomenon. Only then can you claim to know. Many scientists and philosophers have demonstrated that there is no experience you have ever had or event that has taken place that does not have possible alternative explanations. Therefore, you are not justified in claiming to "know" that God exists. You can believe it, but you cannot know it. Is the God hypothesis possible? Yes. Is the naturalistic hypothesis possible? Yes. Therefore, no one "knows".
The rest of the argument is about likelihoods and probabilities. I personally feel that the likelihood of a god existing is very low, but I am ok with believers believing that it is high. But, I really would like those who claim to "know" that God exists know that they cannot know and learn to separate in their minds the raw experience from the interpretation they give to the experience.