There are lots of reasons to believe in God, and I thought a list would be helpful. (Caveat: Whether He is the God of the New Testament is of course a related but distinct question, and some of the reasons below are specific to Christianity and some aren’t.)
One could write a book on many of the items on the list, and defend each at length from anticipated criticisms, but one reason for having a barebones list is that I think it helps make the case for Pascal’s wager. That is, simply seeing the many reasons arrayed together makes a compelling case for concluding that God’s existence is likely enough that the risks and any attendant negative consequences of denying Him are just too great and, conversely, that the chances and any attendant rewards for believing in Him are worth the wager. You don’t have to be 100 percent convinced that each item on the list is true, and you can reject some of them altogether, but if there is a substantial chance that any of them is true, that should be enough. And if you think there is some truth to more than one of them, then pretty soon Occam’s Razor requires you to conclude that it’s easier to believe than to make all the assumptions you have to make to justify nonbelief; to put it positively, you can conclude that a lot of things make more sense, that everything falls into place, if there is a God.
The list follows.
1. There is very strong evidence that the Bible is, certainly in its essentials, historically true — and, in particular, that the New Testament account of the life, death, and resurrection of Jesus Christ is historically true (I’ll just note here the famous “trilemma” subpoint famously made by C.S. Lewis).
2. There’s the Kalam cosmological argument, namely that the universe had to have had a beginning, that the beginning was caused by an uncaused cause, and that — when you think about it — this uncaused cause has all the characteristics of God (it relies on nothing for its existence, has the power to create something from nothing, has the will to do it or not do it, and exists outside of creation).
3. There are St. Thomas Aquinas’s cosmological argument(s) from motion, causation, and contingency, namely that there must have been a Prime Mover, that all the effects we see in the universe must have had some Prime Cause, and that all the merely possible and perishable things we see require a Necessary and Imperishable Being in order to come into existence.
4. There’s the Leibnizian cosmological argument, namely that for anything in the universe to exist, it must have a reason outside of and prior to its existence, but there must have been some explanation outside of the universe to begin with, and again this being is what we call God.
5. There’s the ontological argument, first stated by St. Anselm, who defined God as “that than which nothing greater can be thought,” and argued that this being must exist in the mind, even in the mind of the person who denies the existence of God, and, if the greatest possible being exists in the mind, it must also exist in reality since, if it exists only in the mind, then an even greater being must be possible — one which exists both in the mind and in reality — and therefore, this greatest possible being must exist in reality (I didn’t say that these would be short sentences).
6. It’s extremely unlikely that the many incredibly finely-tuned physical conditions necessary for a functioning universe and for life (force of gravity, electromagnetism, etc.) would all come about without God’s hand; similarly, if everything began with a Big Bang, this suggests a Big Banger.
7. It is extremely unlikely that unguided, random natural selection can explain all of the extraordinary and highly sophisticated plant and animal features and behaviors we find (this includes the “intelligent design” argument).
8. The innate moral sense we have cannot be adequately explained in the absence of God (C.S. Lewis makes this point in Mere Christianity; it’s also made by scientist Francis S. Collins).
9. It’s extremely unlikely that all the accounts of life after death in near-death experiences are false.
10. Similarly, if any miracle (or other supernatural event) is true, then that is powerful evidence that God exists.
11. And, again similarly, if there is a pattern of fulfilled Biblical prophecies, that that too is powerful evidence that God exists (Blaise Pascal stressed this point).
12. Consider the undeniable and dramatic impact that accepting Christ has had on the lives of so many people, and ask if that would be likely if Christianity lacked all truth.
13. There is the sehnsucht (German for “longing” or “pining”) argument, namely that we would not have the deep spiritual longings that we do if there were not truly something to long after, just as we would not feel hungry if there were not food nor thirsty if there were not drink.
14. Somewhat related is the argument that feelings of deep love, outbursts of laughter, and the emotions stirred by music and other great art are not well explained materialistically.
I want to acknowledge that I borrowed from Doug Powell’s Guide to Christian Apologetics and Wikipedia in summarizing the cosmological and ontological arguments. Mr. Powell’s book is very handy and well done.
A number of books discussing the reasons from science (the universe’s fine-tuning and intelligent design) are found in the apologetics subsection of the books section of this blogsite. Books arguing for the historicity of the Gospels in particular and the Bible generally are listed in that subsection of the website, too. I should add that Doug Powell’s book (see preceding paragraph) includes a third science-based argument (“Information as Design: Information Theory and DNA,” pages 57-63).
Finally, if you have had a personal revelatory experience — a reason for believing of an entirely different order — then this list will help confirm that you weren’t just imagining it all.