I said : I don't understand why people blindly believe things that were written hundreds of years ago, without question. I didn't say 'Faith' was blind belief. I was talking about believing in something written by people a long time ago in a bible or holy book
It's a good question. It does seem a bit daft, really, in many ways. I guess it's to do with those people's experience bearing out what is written in those books. For me, it's not about a 'blind' belief in random words written thousands of years ago, that would be akin to a kind of 'blah blah blah, I'm not listening, I believe it anyway' kind of thing, the same kind of faith children who are growing up show when hanging onto Santa Claus. It wouldn't be faith made up of any kind of substance. For me, it is about taking those words, studying them, pulling them apart, exploring the context, discovering the richness of hermeneutics and exegesis, criticising my belief constantly, and my experience being such that I am convinced. I do not think there is no evidence for God, but that's another thread 
As to the OPs question, I find that really interesting. My dad would always have said he would never believe. he thought religious folk were just stupid and deluded, he mocked them and his scientific bent tended to keep him hugely sceptical. Then he had a very tangible experience of God, and was left in no doubt. He became a Christian, and over the years our whole family did. Was I always going to be a person of faith because my dad converted from atheist to Christian when I was two years old? You couldn't say it was in my genes. You may say, well, they brainwashed me then when they became Christians, but that wasn't my experience. We were always told firmly we could believe what we want. It was our choice. When we had stages of rebellion against church etc nothing was said, we were simply left to while away sunday mornings in bed. No sense of being ostracised. Communication was open and honest. I try to do the same with my dc.
Many, many people decide to believe and would say that they never had a shred of belief before. For some it's sudden, like my dad, for some it's more of a journey. I'm not sure we can say that for all people it's innate. Can something so innate only manifest itself at a certain point in life? Surely it would always be there, always be evident even from childhood - but my atheist turned Christian friends would deny that they had such thing as a faith gene and would say they came to faith through a variety of factors - some experiential, some rational, some more spiritual/supernatural.
I think for some, though, it probably is more innate. Some people seem more 'spiritual' - and this manifests itself in all sorts of ways, whether through faith or through something like exploring psychic experiences.
For me, I can't say. I've believed for a very long time; in that time I've had wobbles, some bigger than others. All of them have contributed to the sort of faith I have, and to my quest for exploration of my beliefs, my interest in always reading round on all sides of debates about faith. Is it innate in me? I'm not sure. I know it's real for me. Very much so. But that's borne in experience and reason, not simply a simpering kind of blind faith in some sky fairy or other...