Originally several decades ago, the idea of multiculturalism, was sold as the idea that the West could bring in unlimited numbers of non-whites with a different culture, traditions and religion to the West, and all would still be swell for liberal democracy. The fundamental institutions would not change as race and ethnicity did not really matter in the end, and all were interchangeable in the great global supermarket place.
However, to take but one important group, and to focus upon Europe, the numbers of Muslims have increased over the years to be in 2016, around 5 percent of the Western and Northern European population. At that time France led the way with 8.8 percent, Sweden 8.1 percent, the Netherlands 7.1 percent, the UK 6.3 percent, Germany 6.1 percent, Austria 6.9 percent, Switzerland 6.1 percent, Norway 5.7 percent, Denmark 5.4 percent, Italy 4.8 percent, and Spain 2.6 percent. More recent figures are not readily available, primarily because the political class no doubt does not want to stir up the locals. We can be certain that the percentages are climbing, given mass immigration and the higher birth rates of Muslims.
The issue here is that as the numbers of a different people and religion rise in society, there is a push for the rejection of parts of the culture they hate, and that is a large part of liberal democracy, if not it all. Dr Peter Hammond in his book Slavery, Terrorism and Islam (2005), argues that after a certain critical mass occurs there is a movement for sharia law. As he says: "Islam is not a religion, nor is it a cult. In its fullest form, it is a complete, total, 100% system of life." "Islam has religious, legal, political, economic, social, and military components. The religious component is a beard for all of the other components."
https://www.godreports.com/2015/09/how-islam-takes-over-countries/
Thus, we are seeing this process of what Hammond calls Islamisation occurring with increasing talk about establishment of an Islamic caliphate right throughout Europe. Children are converting to Islam in record numbers and supporting a theocratic sharia law as a new basis for society.
The social vacuum left by the Leftist and materialist attack upon Christianity is being filled. And if it continues it will indeed be the decline and fall of the West, to parallel the fall of ancient Rome.