The US is but one example, but the same thing has happened in the mostly Anglican UK, France, Catholic Spain and Italy, Germany, Lutheran Sweden etc.
In the UK, Anglicanism made up the vast majority of Christianity, whereas in Sweden it was Lutheranism, so there were fewer competing Christian sects than in the US. Spain was moderately religious until recently… I am really wondering what it is that can cause such a widespread capitulation to secularism in but a few generations.
Actually, also of note, Poland is one of the most religious countries in Europe, whereas its neighbour, the Czech Republic, is one of the least religious. Both countries were communist, and Poland is probably going to go the way of the rest of Europe in the next few generations, but I am curious as to what we can learn from this.