One of the fundamentals of Kantian ethics is a universal respect for people — not just those of whom we are fond, or those of whom we approve, or those who belong to a group with which we identify. This of course does not mean we should one-sidedly tolerate extremes of abuse. Respect for people is actually an Aristotelian mean. Like all ethical considerations, it requires a bit of thought in the application.
I used to worry about “metaphysical” or theological preconceptions about what it means to be a person. Even now, I would not base respect for people on a theological notion of substantial personhood, which carries too many presuppositions. Rather, I would start from the Aristotelian concept of rational or talking animals, understood as participating in Brandomian sapience.
I actually believe in respect for all beings, period — including animals, plants, and even inanimate objects. At this level, respect just means a sort of general kindness. But Kant was right to note that there is a profound practical difference when it comes to our fellow talking animals. The fact that we can talk to each other and ask questions of one another makes our interaction with fellow rational animals unique. Even under a broad, somewhat non-Kantian notion of respect for all beings, the kinds of interaction that are possible among beings possessed of language are far richer, and entail more specific responsibilities. Kant himself chose to reserve the term “respect” for those more specific responsibilities.