I’ve stayed out of the discussion because most of my stuff is in storage, so I don’t have access to citations. Still, this notion that Christianity requires Christ to be divine has been repeated so many times by so many people that I’m going to wade in without them. But, first, a disclaimer. I’m not a Christian (though I used to be) (now an atheist, or strong agnostic depending on how one defines terms). To me this isn’t a matter of defending the faith. Rather, it is one of accurate history. Understand, then, that “is” below means “is believed” or similar such phrase.
Part of the problem may be the meaning of the word “divine.” Clearly, in one sense, Christ is divine merely as the Son of God. But that isn’t the same as saying that Christ is God (or, if one prefers, a God or a part of God). IMHO, early proto-orthodox Christians did not consider Christ to be God, a God or part of God. He was sui generis, but more akin to angels or prophets. Note that Christ always attributes his miracles to the Father. More importantly, salvation comes not from Christ but from the Father, at the right hand of whom the Son will sit.
It’s important to understand that this isn’t just nitpicking. The Gospels go out of their way to stress Christ’s humanity, not his divinity. Hence the temptation, Gethsemane, the scouging, etc., not to mention little flourishes like the nativities of Matthew and Luke. The reason they do so is that they were written in the context of a major doctrinal dispute with the Gnostics, who argued that, Christ being divine, didn’t actually die on the cross (you can’t kill a god) but only seemed to die. To the proto-orthodox, this was a dangerous heresy because, to them, for Christ’s resurrection to hold out real promise for mortals, it had to be parallel. A real man’s death and a real man’s resurrection.
So, how did mainstream Christianity come to believe that Christ is divine? This happened later, after the Gospels and Paul’s letters had been written. As the movement matured and expanded, two important issues arose. (We’re talking second and third centuries here.) One was, what was this sacrifice all about, anyway? What did it do and why was it necessary? The other was how to reconcile the Father, Son and Holy Spirit with monotheism, which pretty much everyone agreed was the only acceptable model. The answer to the first became vicarious atonement, which requires Christ to have infinite goodness. The answer to the latter became the Trinity, which has defied sensible comprehension ever since.
But, is it fair to say one can only be a Christian if one accepts these later concepts? In all sincerity, I don’t see it. Seems to me that accepting the Gospels as written should be good enough. And there have been several splinter sects over the centuries who did just that.