The Christian tree has its roots in Jewish ground (think Jerusalem, Athens, Rome).
Christianity built European civilization, especially that of Western Europe, from which American civilization and culture derive.
There are superb and distinguished historians who have demonstrated this; I mention especially Christopher Dawson, Thomas E. Woods, R. W. Southern, Hilaire Belloc, and Régine Pernoud.
This culture began to fragment about 1500; see Jacques Barzun's "From Dawn to Decadence"; and the fragments have largely dissolved.
There came to be a spiritual and moral vacuum in western Europe, which was filled by the gruesome and bloody regimes of National Socialism, Fascism, and Communism. Those were defeated but the moral vacuum remained.
This -- in my mind -- is the main reason Islam has gained such a foothold in Europe, a foothold that may well be impossible to stop or reverse.
In short, insofar as Christianity goes away, secularism, totalitarianism, and Islam will take its place.
And don't think it can't happen here.