No, Christianity is not the only one. Islam, Hinduism, and Buddhism all accepted it.
Christianity was most certainly used as an excuse (see “slave Christianity” in the same reference), but so was Islam.
Racism itself did not “invent” race. In fact, the case can be made that racism began as a survival trait. Think on this: it’s the year 1250 (or even 5000 years before that). You and your family are living on the steppe in central Asia, and you hardly know anyone beyond a ten-mile radius. You see a group of riders. They look different. They have slanted eyes and their skin is not like your own. Are you going to assume they’re regular people, or are you going to assume that they’re probably very bad people (Mongols) coming for your wife and daughter?
In those days, anyone who looked different could be and usually was suspect. In fact, that sentence applies even now, doesn’t it?
Like I keep saying, I’ll accept that whites are the most racist and commit the worst racism in America. No argument there at all. But times will change and the great wheel will turn. Within a couple decades, whites will no longer be a majority…and within a couple decades after that, you’re going to see the national dynamic of racism changing in ways that none of us can foresee. We whites will likely be on the receiving end of a richly-deserved comeuppance. I won’t live to see it, but my sons and their children will.
And that is good.