In response to the original question, “why do mormons claim to be christians” I’m providing this link to the Stanford Encyclopedia of Pholosophy which is clearly not a mormon source. If any of you take the time to actually read it youwill notice a few facts about the history of Christianity.
The earliest desciples of Jesus Christ did not understand God as a “trinity” as he is viewed by most Christians today. Their veiw did not have a oneness in essence or substance, but a clear distinction of three seperate persons.
Even when the word “trinity” started being used (190-200 ad.) the understanding was still of three seperate persons with God the Father having supremacy over the Son and the Holy Spirit (subordinationism).
The ideas about God being three equal, homoousius, were developed not from the bible or from tradition handed down from the apostles, those ideas were developed from concepts pioneered by ancient Greek pholosophers like Plato and Aristotle.
Mormons reject the “trinity” and still call ourselves Christian because we assert that the concept of a trinity was not taught or believed by the earliest Christians, history bears this out.
We also believe the the trinity concept should not have developed over the centuries by way of Greek pholosophy.
If you say that mormons are not Christian because we do not believe in the trinity, you are also rejecting the earliest christians who overwhelmingly worshiped three seperate beings.