Large study on Christianity in Western Europe

That doesn't surprise me honestly, and there's a lot of factors here that have contributed to this.

1) Unlike America where Church and State are (in theory) separate, in many parts of Europe even today they are one; and there have been many, many Governments using religion as chief propagandist and supporter of a wide variety of regimes with dubious reputations (Slovakia's Tizo or modern day Putin come to mind), or in some cases even the reverse where the government has been the pawn of a very shady religious faction as was the case in Ireland until the early 70's.

2) In several countries here, Germany being the worst offender; to actually be a member of any religious organization you have to pay a tithe of anywhere between 8-10% (the "Church tax" which goes to the religion you sign up for). It's a lot cheaper to not be religious in a place where 10% of your income could be sapped for a service you were never going to use outside weddings and funerals.

3) Scandals. That's not to say there wasn't child abuse cover ups in the US, of course there were; but nothing of the range or scale as found in several European states where in some cases evidence had come to light they'd been covered up as far back as 1500AD. Being religious is in the more secular parts of Europe like the UK, Sweden, Netherlands etc religion is not seen as a virtue; if anything it's probably a negative attribute of your character, though one politely pandered to, a bit like being a smoker or too fond of the bottle.

Religion is something that has been used for a good excuse for a war here for centuries, or just to hurt people you don't like. A misuse perhaps, but a use that wasn't seen as much over in the US as it was used...And in fact is still used for here today in some parts of Europe frequently.