Of note, the word "prostheltyzing" in the title is not a misspelling. It means, "Beating someone over the head with the crutch of religion".

Anyway, someone asked me, why I don't believe in (their version of) God. It got me to back up and think just why that is. Here's a partial list (in no particular order):

1. Lack of evidence. You'd think that if there were a God Who really thought it was important that everyone believe in him, that he could, omnipotent being He is said to be, once - just once - give a shred of evidence for something that is not adequately explained through natural means.
2. Prayer does not work. See the double-blind experiment on hospital patients described at  http://www.sciencedirect.com/science/article/pii/S0002870305006496  In fact, this experiment shows a negative outcome of being certain that you are being prayed for.
3. Religion is very divisive. It creates another "us" and a "them" group. Like many instances of having identifiable groups which one does or does not belong to, violence and economic rivalries pop up between the two groups. The groups based on race or nationality have this happen a great deal. Probably not so much though as religion, which too can also identify itself partly as a race or nationality. Violence, oppression, discrimination always break out between such groups, unless it is actively stopped. Religion puts in an artificial boundary between people, and it even more certainly erupts into oppression and violence - including war. This is partly because, unlike such things as race or nationality, being of "the wrong" religion is sometimes or often viewed as "headed straight for hell" by other religions, who may even have teachings to "cleanse" the world of other religions making more way for their own religion.
4. Unlike other forms of discrimination, it is socially acceptable to form clubs, and distribute literature calling for anything between death and discrimination of certain groups. When applied in any other arena than religion, this is deemed "hate speech", sometimes frowned upon and in other times and places punishable by law to have it, write it, or distribute it.
5. Doesn't it seem rather troubling when members of several competing groups eagerly awaiting the end of the world and eternal paradise are in control of powerful, wealthy nations and have weapons of mass destruction at their disposal?  I find it troubling that people who might do this would view themselves as "doing God's work" as they pushed the button or gave the order.
6. I hear a lot about "doing God's work". If God is omnipotent, can't He do His own work?
7. I was not raised a Christian. I encountered it as an adult. Even when I accepted Christianity for a few years, it seemed more than slightly odd to have a God Who came to earth as His own son, was killed, was resurrected, and instructed others to eat his sacrificial human body and blood. Ick! It's still rather bothersome to accept even symbolic cannibalism.
8. Religion of all types asks its believers to accept and believe all sorts of odd notions, without evidence, or upon the evidence of writings done thousands of years ago or upon the pronouncement of a minister, priest, or other cleric. This opens the doors for all sorts of even stranger notions, and even notions which are dangerous to the person and to others flourish. For instance, The Son of Sam murdered many women because he thought God was talking to him through his dog. Had he not been told of God speaking to people to do His will, He might have questioned this and gotten the right kind of help to prevent him from murdering before it happened. The Heaven's Gate and People's Temple, as well as many other cults have ended up with masses of people committing suicide. If these strange notions were not accepted within society, those cults could never have developed such a loyal following.
9. Religion always stands in the way of technological or scientific progress. Such impediments involve laws preventing a particular type of scientific research, teaching modern scientific theories or hypotheses in favor of a Bronze Age mythology, or sometimes killing the scientists, researchers, or people using the technology.
10. Schools, particularly science classes or textbooks, and somewhat social science or history textbooks, as well as literature or other books used for teaching,, have become a debating ground for followers of a particular religion that do not want certain scientific theories taught - to their children or to other children. Schools cannot be debating grounds for such things, or much of the funding, and time of the staff will be spent appeasing the debate rather than teaching the subject at hand. As I'm writing this as a US citizen, this will likely cause the United States to fall farther and farther behind the rest of the work in education, thus science, mathematics, and technology of all sorts. It will effect the literacy rate, which is already appallingly low for an industrialized country as wealthy as the United States of America. As a resident of planet earth, this prevents the best minds from having access to all of the information available, which could be used to solve global problems such as epidemics, pandemics, climate change, population management, agriculture, and creating some sort of sustainable energy.
11. Some of those stories given in holy books, including but most especially the Bible, Talmud, or Koran are quite troubling in their "moral lessons". Such as, God getting angry at humans for sinning, when he knew in advance of creating us given His omniscience we would do, and chose to create us with Free Will.
12. Logical inconsistencies. God cannot be simultaneously all good, all knowing, and all powerful. See "The Problem of Evil" http://www.infidels.org/library/modern/nontheism/atheism/evil.html  You cannot blame it on Free Will either, since God also gave us Free Will, according to theists. If God created Free Will, knowing that sometimes we would use it to create evil and suffering, that would be like a parent who watches as their toddler drinks poison rather than attempting to stop the child. If the parent were questioned later about why they allowed their toddler to drink the poison, knowing that it would cause injury or death, and the parent shrugged and said, "I am not a controlling person. The child was another person, and they have the freedom to decide what to eat or drink for themselves", DFS and the District Attorney would be all over them for child neglect and child endangerment. Why would anyone expect less from a divinity than they would from any human being?
13. Religion causes a great deal of waste. Doesn't it seem rather obscene to have an ornate church, filled with items of gold and jewels, a priest wearing fine clothes and doing nothing but studying the writings concerning God and living in luxury while the people in the surrounding village live in abject poverty, face starvation, high death rates, dirty water, all the while this church continues to take money from these poor people and buy even more gold? Isn't it just as appalling that the ancestors of these people were virtually enslaved to build this ornate building, while their own shacks crumbled, their crops were diminished, and their families died from their bad standard of living?
14. Absurdities in religion abound. It opens way for charlatans of all sorts to take from people power over their own lives, their lives themselves, their money. Asking one to suspend reason in the name of religion allows these things to happen. Besides, if God DID create the universe and everything in it, did that deity not also give us reason to use? Why would He not want us to use it regarding religion as well as everything else?
15. A great number of people, and time from humans, is spent on theological arguments, theological "proofs", worshiping God, and figuring out what God should want applying Bronze age rules to 21st century inventions. This time could be better spent on nearly any endeavor.
16. The Gods in nearly all books are so arbitrary on numerous things, and with so many threats if you do not do these things which seem to be absurd or abusive. Many seem to be the requirements or ramblings of an utter loony. Such as why God will send someone to Hell, without there being any evidence for any part of this. And, God is unable to forgive, which makes Him more limited than nearly any person I know. The tales in the Bible of God getting angry, of killing people, or being a "jealous God" (Jealousy is a sign of insecurity). ordering the enslavement, rape, torture, genocide of so many people indicate that this God is not worthy of worship. So, even if I did believe in Him, I would not worship Him.

While this is not a complete list, and there may be disagreements on some of the points, and I may have written some of them badly or incompletely, they are all above why I should believe in God, let alone some particular understanding of God. Appeal to Pascal's Wager, which I consider to be quite immoral and intellectually dishonest aside, there is another reason to refuse to believe in any God.

(Someone) says to believe in (their version of God) or I will surely go to Hell, and appeals to Pascal's Wager. Even if I accepted Pascal's Wager, there's still a problem. That problem is that there are more than one of these religions that insist if I don't believe in their God, I will go to Hell (or some version of it). The odds of me picking the right one of these where there are X of these religions is 1/X. A large number of these religions exist, and an infinite number of such religions are possible. Thus, as X approaches infinity, the odds of picking the right one approaches 1/(infinity), the limit of which is zero. Thus, there is no chance of picking the correct vindictive religion, so it's a given that one is going to Hell if any of these religions are real.

There are religions that do not send one to Hell for disbelieving in them. They will send one out of existence for not believing as they do, or send them back for reincarnation to learn more to accept the true ways. Believing in one of these has no benefit: Even if it's true, it's only a matter of temporary inconvenience to not believe in it, and after the "re-education" you will receive, you will not be accepting without evidence.

Of course, believing because of personal revelation is probably the least compelling reason to believe something. If it were my own personal revelation, depending on content, it could be a dangerous and absurd belief (See #8 above), or it could be my imagination. As a rational being, if I saw the Invisible Pink Unicorn in my own vision, and She told me that I must partake of Her sacrament of low-fat strawberry cheesecake (for which She gave me the recipe for inclusion in Scripture) every 3rd Thursday, it probably would not do any harm unless I were diabetic or such. If it told me to tell everyone that they MUST partake of low-fat strawberry cheesecake, it would be both dangerous and absurd. Some people cannot get the ingredients. Some people are allergic to some of the ingredients. Some will have bad medical reactions to some of it (such as diabetics). It could injure or kill some people trying to get the IPU's divine sacrament. It's generally not a dangerous sort of revelation, provided I do not apply it to everyone in the world. However, if it told me to get others to arise and kill "them" and take over money and power from "them", and gave me the recipe for black powder or atomic bombs for inclusion in scripture, it could be quite dangerous.

Note that all religions, even major religions, were at some point personal revelations. There is nothing to make it more valid because such personal revelation happened 5000 years ago than if it happened yesterday.