You can call yourself whatever you would like. Just because you don’t go to a specified church or align yourself with a certain denominations beliefs doesn’t mean that you aren’t a Christian. Who really cares about labels anyways? You know what you believe and that is all that really matters. If a person is hung up on what to call themselves and really needs a title to identify themselves then how about “non denominational Christian”…or just leave it at “theist”, of course there are a lot of people out there who will need you to define that term. You don’t have to take part in a group to call yourself a christian or theist.