1. Humans have a well-documented tendency to read intention into all kinds of inanimate objects and systems. For instance, before thunder and lightening were understood to be the result of electrical discharge caused by friction between air molecules inside clouds, the ancient Greeks believed that Zeus was angrily hurling down lightening bolts from high atop Mount Olympus. (Notice how religious myth imitates life: throwing the javelin was a big deal in ancient Greece, and is still an event in the modern Olympics.)
2. Every ancient culture invented its own pantheon of supernatural beings: gods, goddesses, demigods, demons, angels, sprites, spirits, ghosts, goblins, for every occasion and circumstance. The old deities number in the thousands. Now ask any Christian today and they will tell you straight out that these were all false gods, false deities. They're right about that! That Christians don't toss their own God on the trash heap with the rest is a testament to hidebound religious chauvinism.
3. To date there has never been one shred of independently-verifiable evidence indicating the existence of a single god or of anything purported to be of supernatural origin. It's all based on faith. And everything reported to be supernatural is eventually found to be a completely unsubstantiated claim, often in the form of a deliberate fraud (e.g. blood-red tears flowing from the eyes of a statuette of the Madonna, which on close inspection was found to be equipped with hidden spaghetti tubing, a reservoir, a pump...).
4. In spite of efforts to educate the masses, religious belief persists because older people are attached to traditions, and they indoctrinate the kids before they can walk or talk. Imprinting at a young age is extremely powerful in shaping human thought patterns.
5. In spite of their indoctrination, some people do manage to pierce the veil of bullshit, and break free of their mental bonds. This is greatly facilitated by the fact that so much that is asserted in the religion is illogical, unsupported by evidence, and simply doesn't add up.
6. Christianity in particular is full of internal contradictions. For instance, the Book of Genesis has God creating things on days one and two, before he supposedly created the sun. Well how can you have a day without a sun? One Christian believer will say that days one and two were not normal days but special, God days. Another will say they were regular days, but God is great. Biblical interpretation is not very different from reading tea leaves, or casting dry chicken bones on a table top. Disagreements about what the Bible says abound in Christianity, hence the frequent schisms that have produced over 40,000 Christian sects to date.
7. If the God of Abraham is real, then he's one mean, cruel, heartless, son of a bitch. We don't have to look far to see shocking and senseless suffering of innocents. Why, if God is all-knowing, all-powerful, and always present, does He allow this? It doesn't add up. The Christian believer will tell you that "God has a plan," but this is just another circular argument leaving us with no answer.
8. So-called religious experiences can be explained in purely psycho-neurological terms. People with temporal lobe epilepsy report "religious experiences." So do people having a psychotic break, and people with schizophrenia, and people taking hallucinogenic drugs, or simply experiencing severe stress. The fervent desire for such an experience can be enough to trigger it.
9. Religious leaders sell belief to the gullible for profit. Televangelists rake in billion$ every year.
10. Political leaders give lip service to religious beliefs to get elected, or to maintain power, or to justify their actions. Karl Marx called religion the opium of the masses. Those in power use religion to control the masses.

In my extreme youth, I was more open to the possibility that all the talk of God was not pure balderdash, but a lifetime of studying science has clarified my thinking: supernaturalism is bullshit. There is still some mystery around the nature of dark energy and dark matter, but science does show that, at least as far as our planet and solar system are concerned, no supernatural force is required to explain everything we see in nature. All chemical elements heavier than hydrogen were forged in the cores of stars through gravity-driven nuclear fusion. Planets form by accretion. And the evidence supporting Darwin's idea that all life on Earth evolved over billions of years from a single, relatively simple ancestral species is quite literally overwhelming.