I think that belief in 'God' (however that word's defined) is a religious belief, simply by definition. So anyone who believes in God can be said to possess at least one religious belief. In other words, I'm inclined to think that it's impossible to completely separate 'I believe in God' from 'I have a religious belief', such that it's logically possible for the first statement to be true while the second is false. But it's certainly possible to believe in God without belonging to an established religious denomination. It's possible to be a thoroughly secular person in day-to-day life, while still believing in God. It's probably harder to believe in God without being influenced by the religious traditions that have historically used that word, since those uses have provided the word with the variety of meanings that it bears. But certainly people can and do arrive at their own unique ideas of what they think God is.