I haven’t read all the replies, so forgive me if I repeat a point.
It would really depend on the country in Europe. For example, In Ireland, where the Catholic Church was traditionally very strong in membership, is seeing massive numbers of people leaving the church. In a country of 4.7 million, the main seminary, had 7 seminarians last year.
Then if you look at Poland, you might see a strengthening of certain Catholic practices. But again, as they become more western, people tend to leave the church.
That being said, an interesting phenomenon, in European Christianity, is that the more Orthodox branches of Christianity. Being Eastern Orthodox and the like as well as Catholicism, are seeing an upsurge in numbers in the last few years. For example, if you take a trip to a cathedral in Paris, you may be surprised to see many, young Lawyers, doctors, Business men and Engineers, mainly men, populating the pews.
The same is the case in eastern europe, where the Eastern Orthodox Church, in places such as Romania has seen an explosion in young people attending their services.
To add to this, in recent years, the number of Adult Baptisms in Catholicism, as begun to grow in the last couple of years, as have the number of reversions. The biggest demographic of christian converts across Europe is coming from Islamic migrants, who have begun to enter the countries.
From what I can see over here, in Europe, we are approaching the lowest points of Christianity in Europe, with regards to shear numbers. However, the Catholic community that is beginning to blossom, across Europe is a far stronger Catholicism than we have had in many decades. The same goes for Eastern Orthodox, even more so.
It has led me to the conclusion that there is something driving this increase over the last few years. If you ask any recent Revert or Convert, you might get some mixed approaches to views. But you will see a resounding amount of homogeneity. Young men especially, have become enchanted with the ideas of traditional christianity. Of the many, new and recent converts to Catholicism that I know of, nearly all enjoy latin in the mass. Nearly all are critical of the interpretations of Vatican 2 and most, are beginning to reject the weak pandering, to modern day cultural ideas, that we have been seeing for the past 60 years.
In short, Christianity is indeed, still decreasing in numbers of faithful. But lack of structure, meaning and discipline in many young men, is leading to a backlash against these ideas, in search of true meaning. Which is why, seemingly, the more orthodox the christianity, the more pious the followers tend to be.