A secular society is one such as all Western societies, where public discourse is not framed by a religious perspective and priorities.
In such a society mass immigration is a win-win strategy for the demonic powers because in itself it will (by various means, both direct and indirect) tend to subvert residual Christianity - but perhaps most powerfully by being both symbolic of the underlying despair of secularism, and in its effects encouraging an enactment of the 'death wish' for cultural suicide and individual self-annihilation.
The win-win aspect is because in public discourse secular opposition for mass immigration in this context can only be based upon material and psychological selfishness (since that is the only possible bottom line basis of all secular morality which denies the 'transcendental realm) - in other words, opposition to immigration is based upon the wish to preserve (or enhance) personal wealth and this-worldly happiness. Therefore it will attain political motivation and power by encouraging variants of greed and hatred.
And - a neglected aspect - mass immigration to The West is a win-win for Satan (in the large majority of instances) because it is economically and psychologically motivated and leads to the corruption of the immigrating population by secularism (eg. the evil effects of a pervasive mass media, the destructive effects of economic dependence, and a victim mentality which is deliberately inculcated by The Left in host nations).
In sum, mass migration into the West destroys both Western Christianity and the Christianity of the migrants.
However, a distinct category is the other factor in world population movements, which concerns what might be termed religious migrants, who remain observant or even intensify religious observance in The West; and who seek to expand their religion into The West.
It is interesting to speculate, from a Christian perspective, why God has allowed this to happen - and the most obvious answer is that (to God) any religion is preferable to no religion; any religion has more potential for human salvation (we are not primarily talking about human health, wealth or happiness) than the currently dominant Left-secularism of The West.
So, given the mass apostasy and anti-Christianity of The West - and its subversion of even Pagan common sense and natural decency - the best available option is some other religion rather than the utter despair and inversion of The Good which is characteristic of all secularism on average and in the long run.
None of this is inevitable. If the West chooses to embrace Christianity again, then (and only then) the Western Peoples would find the loving motivation to deal with the phenomenon of mass immigration in the only non-Satanic way it can be dealt-with - by working not for human mortal comfort and convenience, but from the eternal and divine perspective.