If one has A Christianity (or an interpretation of the Bible) that does not contradict any part of the Bible then their Christianity is as valid as any other Christianity that also does not contradict the Bible.
Many of the mainstream Christians actually do not mind contradicting the Bible and claim that the parts they are contradicting are either no longer relevant, or were not inspired, or they say something like Paul was talking to a particular audience, Paul was mistaken, or they perform great mental gymnastics to come up with an extremely long reason for why the text actually says the opposite of what you think it is saying.
Many people agree with these misguided mainstreamers because they do not wish to lose the label they have chosen for themselves: we are Calvinists so we'll stick together, or we are Catholics and what is the current popular interpretation is the right one and we don't need to spend our mental energies to figure out whether these interpretations hold out or not, we'll leave that to the doctors of the church. People are extremely conservative when it comes to spending their mental energy.
Some Atheists are good at pointing out these contradictions and rejecting these misguided claims but these Atheists don't go far enough, as in they do not come up with their own interpretation of the Bible. Why don't they? Because they do not think that Bible is worth their time and effort. And it would take a LOT of time to read and re-read the Bible before you can get a good grip on the meaning. However, the Christian scriptures have been the most important text in human history and countless people have spent their entire lifetimes studying the text and refining their interpretations. And they have gained extreme joy out of this work.
Bible asks us to call no man our father on earth, which can mean accept nobody else's interpretation if it doesn't suit your mind because nobody else on earth has the final say on what you believe or do. You should be able to completely own your interpretation of the text. In other words you shouldn't blame somebody else for giving you a misguided interpretation because you yourself will be held responsible for your actions. Bible also says that the road to life is narrow and only a few find it, whereas the road to destruction is wide and many enter through it. Choose the narrow path and don't be led by misguided people.
although I don't think that there is a reason to believe it
You may think that there is no reason to follow the two commandments but if everybody were to follow them it would lead to the best outcome for mankind. Kant said that you should act only according to that maxim whereby you can at the same time will that it should become a universal law without contradiction. And these two laws are the best maxims to live by.