While I personally use reasoning in determining what I believe, it doesn't seem that way for most humans. If religion was not primarily influenced by culture, we wouldn't see 95% Christian in USA, and only 5% in India and China. It seems that indoctrination and a sense of pride in cultural heritage leads people to believing in things, rather than a persons reasoning, which many seem to reserve for other subjects.
If you were bron in America, chances are you were born a Christian. If you were born in the Middle East, chances are you follow Islam. The only time I have seen somebody be a religion different than that of the culture they were raised in, is if they decide to be Wiccan, Buddhist, Agnostic, or Atheist. I don't understand the Wiccan religion myself, but Buddhism, Agnosticism, and Atheism are the more rational and scientific "religions." All Buddhists are Atheist.
Basic statistics proves this point. The VAST majority of people in this world are whatever religion they were raised to be and what is culturally prominent in the area they live. It's why you don't see a nice even distribution on the world map of religions- Hindus are concentrated mostly around India, Buddhism in Asia, and Christianity in the Americas (fading in Europe).
Many nations use religion as a sense of identity in ones culture or ethnicity. American people are predominately christian especially in the bible belt south and many of the latin american nations and mediterranean europe are predominately catholic mainly due to cultural upbringing. The middle east are predominately muslim and that comes from culture a forceful upbringing. Though i cannot speak for everyone, many of the people are forced into worship due to sharia law so i guess this does not apply to everyone in the middle east, but trends do show that many people follow a religion just because it is the only culturally acceptable way to live in their environment.
It's not unreasonable to assume that the religion of choice for an individual is largely influenced by that individual's culture of origin. Christianity has been the religion of choice in the West for thousands of years, as well as those countries which were colonised by Western nations. Same goes for Buddhism in the Far East and Islam in the Middle East.
However, don't be fooled into thinking that just because one's 'god' is influenced by one's culture, this sufficiently refutes the existence of a god.