By Matt Slick
We can know that God exists because without him we cannot make sense of anything. Let me explain.
To be rational we must utilize the laws of logic, but those laws are conceptual statements that transcend space and time. That means they are true no matter when and where we are. They are not dependent on space and time for being true. Also, they are conceptual absolutes because they are truth statements. These laws are the Law of Identity (something is what it is and is not what it is not), the Law of Non-Contradiction (a statement cannot be both true and false at the same time), the Law of Excluded Middle (a statement is either true or false), and the Law of Proper Inference (if A = B, and B = C, then A = C), etc. These laws are not dependent upon chemical reactions in the brain (chemical reactions don't produce logic). They do not obtain their universal truth values because we agree upon them (otherwise they can become false if we decide they are false). They cannot be properties of the universe because properties can be measured, but these conceptual laws cannot. Therefore, since materialistic-based explanations can't account for the laws of logic, a non-materialistic based explanation must be true. Also, since the laws of logic are conceptual, it makes sense to say that the non-materialistic explanation is a transcendent mind, i.e., God. Without a transcendent mind, how can anyone justify transcendent conceptual absolutes? It does not seem possible.
When we consider our own existence and we look back at what caused us, and what caused that cause, and so on and so forth back in time, until we ask where the universe came from, we run into a logical problem if there is no God. In order for an event to occur it must have a cause, but we cannot have an infinite regression of causes because that would mean there is no first cause. Without a first cause there cannot be a 2nd, or 3rd, or 4th, etc., up to now. So, it would have to be that there was a single uncaused cause. Furthermore, if that which is the uncaused cause was material and not personal, then what caused the universe to exist would have needed to possess the necessary and sufficient conditions by which creation would be possible. But if it possessed the necessary and sufficient conditions, then the result what have been automatic and the universe would have automatically been created. Since the cause is before the universe it would have to be without time, because it existed before time came into being. This would mean the uncaused cause would have brought the universe into existence, automatically, in the far reaches of the "infinite past." However, this is not the case since the universe is not infinitely old. Therefore, a non-personal cause does not make sense. It makes sense to say that existence of the universe was brought into existence by a personal cause. The personal cause has the necessary and sufficient conditions to bring the universe into existence, and being a personal being decided to create at some point in time. This would explain why the universe is not infinitely old.
When we look at morality, we realize that without an objective moral standard we cannot say what is truly right and wrong. All we would have is subjective opinions about what is right and wrong. But people all over the world, even atheists, speak in terms of moral absolutes when they say something "is" wrong. Without an objective moral standard of judgment, no one can say something "is" wrong or "is" right. He could only say that he thinks or believes that something is right or wrong. Furthermore, if a non-theist were to appeal to the opinions of society, common sense, or intuition for the source of morality, then all of these moral judgments are subjective expressions and opinions. In that case, there is no absolute right and wrong and the only thing we're left with is whatever a particular group of people agrees upon as right and wrong... until they change their opinion. This is naturally problematic because if we site a universally true moral statement (such as "It is always wrong for anyone to torture babies to death merely for their personal pleasure"), why is the statement absolutely true? Since subjectivity, opinion, and majority vote don't make absolute statements true, it makes sense to say that an absolute truth statement about morality requires an absolute moral lawgiver: God.
In the three most philosophically important categories of epistemology (how we know things), we see that without God we cannot make sense of rationality, our existence, or morality. It makes more sense to say that God is the author of the laws of logic upon which rationality is built. It makes more sense to say that we came into existence by a decision of a supreme being. Finally, it makes more sense to say that absolute morals require an absolute mind.
So, is there a God? Absolutely, because without him we can't make sense of anything.