There are a number of reasons why we know that there is a God, and we will briefly examine 5 of these. In Acts 17:22-34 we see arguments used by the Apostle Paul through the inspiration of the Holy Spirit that are common to our apologetics in answering the question “How do we know there is a God?”
Paul was in the market place (the Agora) of the city of Athens. He entered a discussion with certain Epicurean and Stoic philosophers which resulted in their going to the Areopagus on Mars Hill where they could discuss this matter more in depth. The Apostle Paul began his argument by pointing out the altar to the Unknown God, this is the one that Paul will explain to them. He begins with “God who made the world and everything in it, since He is Lord of heaven and earth, does not dwell in temples made with hands.” (vs 24). This is the first of our arguments and is known as the cosmological argument. It is reasoning from the physical law of “cause and effect” i.e. for every effect there is a cause and the cause is greater than the effect. For example, if a rock came flying through your window, you would instinctively know that someone threw it or it flew off of a passing truck etc. One would never think that a rock would pick itself up and throw itself through a window. On a much grander scale we know, from observing the universe that, everything in the universe comes from something else, i.e. cause and effect. Corn grows because a seed was planted in the ground. We also know that ultimately there has to be a cause that itself is not caused, or as Aristotle put it, there must be an unmoved mover. There is no such thing as infinite regress, there must be an ultimate cause, who is Himself uncaused, and that is what we know as God.
Paul continued his discourse with the anthropological argument, that all of mankind was created by the Lord God, “And He has made from one blood every nation of men to dwell on all the face of the earth,” (vs 26). Mankind did not evolve from monkeys, but was created in the image of God. There is far greater scientific evidence that mankind was created as a special being apart from the animal kingdom than there is that there is a monkey in our family tree. This gives value to all peoples out of every nation.
The third argument is, “…for in Him we live and move and have our being,” (vs 28). This is the argument from being which is, in this text from the word esmen which is that we are, that we exist. This is known as the ontological argument that we exist because God exist and therefore, God must exist. We were created in the image of God and we have being because we were created by the ultimate being. A fourth argument is related to the first one, which is the teleological argument which is from design. This great universe is both immense and complex and as such must have a creator who is greater in both immensity and complexity. Only a fool will think that order can come from disorder, but we all know that order can only come from a greater order and in its breaking down, it becomes disorder.
A fifth argument is that of the moral argument, that morality and what is right and what is wrong must come from a perfect judge. In our text, this is referred to in verse 31, “…because He has appointed a day on which He will judge the world in righteousness by the Man whom He has ordained.” Our Lord has proclaimed that He is the way, the truth, and the life, absolute truth does exist and it exist in our Lord Jesus Christ.
There is much more that can be said on this subject and I will soon put a study on the subject of Christian Apologetics on this website. To even dig deeper there are many books on the subject as well such as Geisler’s book Christian Apologetics.