A common criticism people who condemn the mass immigration of recent years face is that they are unchristian. This has always struck me as a particularly insincere argument, and goodness knows we get plenty of those nowadays, because it completely reframes Christianity into something else entirely. No longer is it a spiritual path centred on the supernatural reality of God who became man in order to open up a way out of the material world for those who would accept it. It is now just a form of secular humanism.
The fact that there is no Jew nor Greek nor male nor female nor free nor slave in Jesus Christ does not mean we are all the same and all equal. It means that all human beings can find salvation in Christ. So it means that there is a kind of unity but the unity is in those who have turned to God through Christ. It is not spoken of as universal for all human beings under any circumstances. Besides, there are still Jews, Greeks, men, women and so on. There is now something that unites them at a higher level but on the ordinary, everyday level they remain what they are. They are not suddenly an amorphous, identikit mass. What is more, that quote from St Paul might be read in tandem with another from Acts chapter 17 verse 26 where he says, "From one man he made all the nations, that they should inhabit the whole earth; and he marked out their appointed times in history and the boundaries of their lands." I am not saying this proves anything one way or the other but if you are seeking to make a point through Biblical quotes you can't just select the ones you like and ignore the ones that don't back up your already formed convictions.
Christianity is concerned with the salvation of the soul. That's all. Christ's kingdom is not of this world. So the immigration question, which is a worldly matter, is peripheral to the core purpose of Christianity. That means that the opening up of a country's borders to large numbers of foreign people to the degree that the country is radically transformed does not form part of a Christian's essential duties. However, the soul that would be saved must clearly behave in a certain way concordant with that, part of which is to love his neighbour. In obeying this commandment great Christians have made enormous self-sacrifices to benefit others. But what is right on an individual level is not necessarily so on a national one, and if you think loving your neighbour means allowing unlimited immigration then you must determine who your neighbours are. Are they the people thousands of miles away you don't know or are they your actual countrymen, many of whom might have their lives severely impacted by the arrival of newcomers? You see, glib generalities are not so simple after all and even sayings of Christ have to be understood with discrimination and wisdom, and not forced into inappropriate context.
On one occasion Christ fed the 5,000 but he didn't do that all the time. He left plenty of people hungry during his ministry. He healed the sick but he didn't heal everybody, and when he was asked why the man to whom he restored sight was born blind, he said it was so that the works of God might be made manifest. In other words, his miracles were mainly to demonstrate God's power and reality. If Christianity really were principally about healing the sick and feeding the poor then Christ didn't do very well. As he himself said, "The poor will always be with you." That doesn't mean don't heal the sick or feed the poor but it does mean don't claim that Christ would automatically say we should welcome all immigrants with open arms, no matter how many of them come. The principal focus of his teaching was spiritual not humanistic. It was heaven not earth.
This debate reminds me of something I read recently on Bruce Charlton's blog where he discussed sexual morality and made the point that sexual sin, although not the greatest of sins in itself, has the effect, if unrepented, of dulling the mind to proper morality in general. See here. In the post by Ed Feser which inspired Bruce's post someone had commented to dispute this point that we are far more advanced in matters of gender equality and racial equality than earlier generations were and this happened after the sexual revolution of the 1960s. Point disproved according to him but I don't think so. He took these things to be moral matters but that just reflected his own prejudices and his assumptions that they are true and that someone holding these attitudes is more moral than someone who does not. But actually these are ideological attitudes not moral ones. Of course, there is overlap, sometimes considerable overlap, but real morality is based on the truth of God as manifested in the order of being expressed through creation. It is not a question of human opinion. Gender and racial equality are not spiritual beliefs, they are political ones. There is certainly no foundation for them, as understood nowadays, in either traditional religion or scripture.
By the same token, the idea of mass immigration being something a Christian must accept, even accept with joy, is not based on anything in traditional religion or scripture. Love and justice are part of the spiritual life but cultural self-destruction is not a religious obligation and those who claim that it is, or that which causes it is, are distorting Christianity and reducing it to a materialistic level to suit a leftist agenda which is primary. Which does not mean that the opposite is true, that Christianity by default must be against mass immigration; only that other criteria must be brought to bear to provide an answer. Bringing comfort to the suffering is a spiritual duty and no religion has approached Christianity in doing this. But you are instructed to love your neighbour as yourself which rather implies that you must first love yourself. Mass immigration is a kind of self-hatred since it will inevitably lead to a nation's destruction as the cultural entity it traditionally has been. It is, and I think has been conceived as such, an attack on a nation's soul disguised as humanitarianism.