Why do some Christians not believe in the Trinity?

From the earliest days of the Christian Church there have been those who did not believe in the Holy Trinity, because there is no scriptural support for the concept. During the fourth century, Arianism, a Christian view that rejects the Trinity, almost won the day and would have led to all modern Christians not believing in the Trinity.

Even today, some Christians do not believe in the Trinity. Whether or not that is the most appropriate belief, it is at least rational.