One can make many arguments for or against the existence of God, but one can also refute or minimize most of them. There is one argument for God, however, than has no flaw whatsoever. It is impossible to refute, minimize, or deny. To do so would be intellectually dishonest. This argument is the cosmological argument, though I prefer to call it the causality argument. It reasons as follows:
The basis of science is observations and testable predictions. One forms a hypothesis to explain or outline visible phenomena. If evidence shows the hypothesis to be true repeatedly, with no discrepancies or hints of error, over a long period, one may call it a law. Every scientific law governs a certain region of the universe, or a certain type of process. There is one law that applies universally to all objects that exist within a dimension of time, all objects that have duration. This is the law of causality, also known as cause-and-effect. It dictates as follows: every event has a cause. This cause is an event, and thus requires a cause just as its effect does. An immediate problem with this should seem obvious. If all events require a cause, this would mean that there are an infinite number of causes going back into the past. However, this conflicts the second law of thermodynamics, which says that the universe is continually winding down. If the universe is continually winding down, it cannot have an infinite extent in the past if our world currently has order. Supporting this is the observation that the universe is expanding, which shows it had to have a beginning.
There is an obvious conclusion. The beginning of the universe would have to be an event. Since causality is a law, it means that the beginning of the universe would have to have a cause. This poses another issue, however. If time began at the beginning of the universe, then how could anything cause the beginning of the universe? Well, the cause would have to exist outside of time. Since space is bound to time, the cause would also have to exist outside of space. What exists outside of time and space? Well, the scientific world is silent on this matter. This is because science is the result of observation and experimentation, both of which are by humans who exist only in time and space. The religious community, however, provides an answer.
There is one Being that has always been conceived of which would be able to be an initial cause. This Being is God. God, not being confined to space and time, would be exempt from the law of causality (remember that causality can only be said to apply as far as it has been tested, which is only within space and time). Thus, He would not need a creator. People often will object, saying that we have no reason to believe it was God. It could have been something else, such as chance. This is wrong. Chance is based on measurements of probability and statistics, which are all defined and observed within space and time. We, then, cannot say that chance can operate outside of space and time. Since the only logical alternative to chance is choice, and choice is the result of intelligence, the universe had to have an intelligent origin. Many people, when they concede to logic, try to leave it at that: they say that we cannot know the identity of the creative intelligence. However, strong logical evidence shows that this Intelligence must be God. For example, choices are based on knowledge or emotion. All choices require some knowledge to be acted. Therefore, the creation of the universe would require a being that knows enough to create the universe. Also seeing that this universe has incredibly fine-tuned parameters for the existence of life, and shows incredible design, it makes sense that this being should be omniscient, a core attribute of God. Next, realize that all action requires power and ability. Since the universe is so massive, and is all we can perceive, this being would have to be all-powerful, or omnipotent, which is another core attribute of God (if you object to omnipotence, see my previous post). Finally, we must remember that any being outside of time has no beginning or end, and is thus eternal, another core attribute of God. The sum of all this, then, is that God must exist. At this point, the causality argument is complete. The law of causality requires the existence of God.
Deists are willing to concede on this point, but they will say that we cannot know the identity of God, that religions do not necessarily describe God, and that God has nothing to do with us besides creation. However, I believe that we can take the reasoning used so far even farther as identify our Creator as the Christian God using creation itself. A core attribute of the Christian God is love. Logic shows that the Creator must love us. How do we arrive at that conclusion? It is quite simple. God obviously had to do incredible fine-tuning of the universe to ensure the existence of humanity. This implies that He made us for a reason. There are only two reasons He would make people: He either loves them or hates them. If He hated us, though, the universe could be much worse. After all, the world, even in all of its sufferings, is a decent place. The world could be filled with much more torment; our lives could be much more miserable. If God created us out of hate, why would life be good to anyone? Why would life not be terrible for all people? There is no reason. If hate is not the reason God created us, then He must have done it out of love. This is a huge, unique quality of our God, the Christian God. There is also more evidence of His identity. For example, the Christian God is a Trinity. If He made the world, it would make sense that He left some tripartite structures and systems as His fingerprints. There are many such examples of this. Atoms are made of electrons, protons, and neutrons. Protons and neutrons are three-quark combinations. The physical universe consists of space, time, and matter/energy (these are two forms of the same thing). Humans have a head, a torso, and legs. The heavens consist of the sun, the moon, and the stars. Matter exists in three main states: solid, liquid, and gas. Time is the present, the future, and the past. Space has three dimensions: length, width, and height. These are only a few of the many threes of the universe. One may argue that this evidence is circumstantial. While it is true that any one of these things could be a coincidence, all together they amount to more than just a statistical curiosity. They seem to identify the Creator as a triune God. If creation shows that God is both loving and triune, and these are the two defining traits of the Christian God, then it is only reasonable to say that the Christian God, Jehovah, exists and is the Creator of the universe.