All Christian denominations will propagate particular ideas about how to interpret the bible. Should Mary be venerated? Should baptism be a prerequisite for salvation? Should people participate in active evangelism? The Roman Catholic Church, for instance, traditionally take a no-holds-barred policy against the use of contraception, while other more modern Protestant churches will tell you that contraception is allowed in a heterosexual marriage. Baptists generally believe Baptism in water is fundamental to salvation, while some Methodist denominations may not.
Christians, by these contradictions, show a fundamental lack of unity, and for a body who collectively claim to hold in their possession the most important revelation in human history, such a lack of consensus on what “true Christianity” is, fundamentally undermines their claims.
Supposing a person does believe in God, in the bible, and in Jesus of Nazareth and the Christian truth. How do any such people know which interpretation is the true one? How do they come to the conclusion that they’ve got it right?
Christians answer these questions predictably: the adherents of a particular denomination will say that their own interpretations are correct. In all my experience I am yet to meet a Christian who doesn’t assert in some way, that they have it right. But never have any of these claims been accompanied by a logical reason why an atheist should believe any one particular person making the claim to correctness, over another. There have been no responses I have come across which reference an impartial mechanism for determining validity. Making such claims on the basis of nothing more than the apparent validity of the claim itself is of course the definition of circular reasoning.
If Christians can't establish which denomination is actually correct, what does it matter if a person chooses any one denomination over the other? For Christians, there are the obvious theological problems with this question, the most obvious one being that, to a Christian, it matters for their salvation whether they've "gotten it right" or not. If they believe a heresy, they might not get to heaven. But if they cannot establish any significant agreement on which teachings are heresy and which aren't, then even if it does in fact matter for a person's salvation which denominational beliefs they hold, it's irrelevant, because they can't establish for certain which denominational beliefs are the right ones.
Further, if Christians can't establish why it is logically preferable to believe in one doctrine over another, then even if one denomination has in fact stumbled upon this supposed “truth”, it is not possible to know it. If a singular Christian “truth” exists then every single denomination except for one must be incorrect, yet Christians have no way of logically deducing which one it is. If it is not possible to define which denomination, doctrine, and beliefs are correct, then denominational preference is nothing but a subjective personal choice. And so, the entire concept of a cosmically correct, Christian truth, is itself an irrelevance to the reality of their beliefs.
Believe what you like, because it really doesn’t matter.