In God. I believe because after careful consideration, I came to the conclusion that there is an all-powerful, wise, loving and just person - not a physical one like humans, but with qualities and emotions like us and action is the origin of these said things and not the other way around - who is responsible for the creation of our physical universe. My conclusion is that there is absolute no reason to think that he doesn't exist and a astonishing amount of prove to believe he does. The simplest one is: things don't come out of nothing. The very definition of nothing is the lack of existence of something, anything that can be called everything.
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Some people affirm that there other reasons why I believe. I disagree with them. Many feel that even mistaken beliefs cause little harm to anyone. Someone who believes that the earth is flat is not likely to hurt himself or anyone else. That's not wise. A doctor that simply agrees to differ if one of his colleagues continued to believe he could go straight from handling dead bodies in a morgue to examining sick patients in a hospital ward, would be irresponsible, to say the least. Mistaken beliefs have caused great harm. The Holy Crusades of the Middle Ages is one example. All these zealots believed that they were right. Yet, obviously something was terribly wrong.
I believe this happens because Satan the Devil is misleading people. (Rev 12:9; 2 Cor 4:4; 11:3) Paul said that many religious people would be doomed to perish because they would be deceived by Satan, who would produce miracles and wonders calculated to deceive. They would shut their minds to the love of truth which could have saved them and be ‘deluded into believing what is a lie.’ (2 Thes 2:9-12) There is a way to minimize the possibility of believing a lie. First there are things that should not be the origin of any belief.
I don't believe things simply because my parents or anyone else believed them - in fact, I didn't know anyone with the same beliefs. Unthinking adherence to what previous generations believed and did can, in fact, be dangerous. (Ps 78:8; Am 2:4)
Many teachers and authorities deserve great respect. Yet, history is filled with examples of renowned teachers who were absolutely wrong. Regarding two books on scientific matters written by Aristotle, Bertrand Russell stated that “hardly a sentence in either can be accepted in the light of modern science.” Even modern-day authorities often get things drastically wrong. “Heavier-than-air flying machines are impossible,” was the confident assertion of British scientist Lord Kelvin in 1895. A wise person, therefore, does not blindly believe that something is true simply because some authoritative teacher says it is. (Ps 146:3; Mat 7:26; 1 Cor 1:19, 20) Human knowledge is very limited. The same caution is needed when it comes to religious education. (Gal 1:13, 14; Jo 16:2, 3; Act 9:1-6; 26:14)
The media can influence beliefs. Most people are glad that there is freedom of speech in the media, giving them access to information that can be useful. But there are powerful forces that can and frequently do manipulate the media. What is often presented is biased information that can insidiously affect your thinking. To appeal to or to attract a larger audience, the media tend to give publicity to what is sensational and unconventional. What could hardly be said or printed for public consumption just a few years ago has become commonplace today. Slowly but surely, established standards of behavior are attacked and whittled away. People’s thinking is gradually becoming distorted. They begin to believe that “good is bad and bad is good.” (Isa 5:20; 1 Cor 6:9, 10) God has given you intellectual capacity to investigate the world around us and to ask questions concerning spiritual matters, so it makes sense that he would also provide the means to get accurate answers to your questions. (1 Jo 5:20; 17:17; 2 Tim 3:16, 17; 1 Jo 4:1)