This turned out to be quite long, and quite indirect before I got to my counterintuitive final set of points. I will have to find some way to divide this for readability.
I speculated in my Reducing Racism post that high levels of immigration may interfere with the acceptance of actual citizens, especially African-Americans. That there is an economic effect is widely believed among African-Americans and is sometimes noted by whites as well. Most prominently and recently, Bernie Sanders has long asserted this and made this a part of his early campaign. However, it doesn't come up as a common cliché in our discussions of either immigration or racism. Relatedly, I wondered if there were a similar emotional effect - and that is absolutely not a cliché of our current discussions. On the contrary, a supposed opposite effect is a cliché, that acceptance of blacks and Hispanics and Muslims and Asians and Natives is all tied together as a white rejection of "brownness." Grim asked if I had any further thoughts about why this is not a cliché, and asked his own audience to consider the matter as well.
I gave a preliminary answer that it might be tied in to a Christian-derived absolutism to accept all or give to all on the basis of need, regardless of background. Not that all Christian groups have taken this attitude - most have continued to be at least partly insular, tribal, clan-based. But this sort of universal brotherhood idea doesn't occur in other groups at all. The idea of some gradualism is clear in the OT. Yahweh declares it's OK to intermarry with this tribe but not with that one, and boundaries are sometimes fuzzy. There are hints all along that Israel has some special purpose among the nations, not to exterminate them or ignore them forever, but to be the vehicle for their eventual acceptance as well.
But I don't know of even gradualism being part of the thinking of other groups worldwide. So. This idea that we mustn't pick and choose, mustn't say "First we'll get all the black people up even, then move on to folks from other countries" must descend at least in part from Christian heritage. Some Christians would consider the rule ironclad, and the idea has certainly been around for the last 2000 years. Yet I don't think anyone but Americans, Canadians...maybe some others in the Anglosphere...some other Europeans...Well, when we get to "some other Europeans" we start to see a different picture. Plenty of folks in the nearly-defunct European churches would subscribe to that kind of universalism, as would their Arts & Humanities tribe, many journalists, government officials and politicians. But notice an immediate difference. Those latter groups may say "we need to be open to everyone," but it is never phrased as "we have to accept Roma, and Jews, and Syrians, and Pakistanis, and Gambians." It is framed in the negative: "We must not be nationalist." Nationalism is the great fear of European elites.
That fear among them goes back well before WWII and the Nazis, though they are the usual excuse. It goes back to Marx, and all the sub-creeds of believers who divided humanity horizontally rather than vertically. Nationalism was the main competitor to Marxism in both the 19th and 20th C's. (Perhaps religion was also an equal competitor to both. Worth an exploration sometime.) Accepting "everyone" is more of an abstract. It is usually pulled out only when the acceptance of some specific group is being discussed. Currently, it refers only to one category of the world's refugees.
As Marxism is something of a Christian heresy right out of the gate, it becomes very difficult for any of us to discern where our belief in universal brotherhood comes from. Even Christians who hate Marxism, and Marxists who hate Christianity, learned their values in cultures where the other was present and influential.
The New Testament is pretty solidly in favor of accepting any who will join the Church as equals: neither Jew nor Greek, etc. (Though also, first to the Jew and then the Greek.) Yet even there the idea of universal brotherhood is not so strong as advertised. Galatians 6:10 Therefore, as we have opportunity, let us do good to all people, especially to those who belong to the family of believers. Do good to all people, but especially… And also 1 Timothy 3 or 5; Matthew 25 says both, specifying generosity to the brethren in the first part of the statement, expanding to everyone in the second. Relatives, families, tribes remain important. In the pictures of heaven in the Revelation to John, everyone remains grouped in nations - they are not an undifferentiated mass. Notice also, whatever universality there is acceptance into the fold - none are to be turned away. But nothing is said about all of mankind being brothers. The translation in Luke is not "Peace on earth, good will to men," but "on earth peace to those on whom his favor rests." (Or older, to men of good will.) Generosity and kindness toward all are required, believer or no. But those in the Church are your new tribe, and deserve your favor even more.
Even though it is weakly attested in the Bible, gradualism in accepting others, including Christian others, is clearly part of church history. While believers of any stripe were welcomed in the early church, once their numbers grew and the Roman Empire crumbled this weakened. The Eastern Church continued to have Councils, but their tribes remained tribes. The Roman Catholic Church made significant advances unifying monasteries and orders across boundaries, but tribes remained tribes in Medieval Europe as well. Accepting people who were not distant cousins was still a long way off.
Arbitrary break: The first line of the second half of this is There is a third strain of equality that I think is not a parent to the American pattern of accepting all without distinction, but an older sibling, and that is equality before the law. So just absorb this for now. I imagine you are as weary as I am.