Dreher has blamed the Reformation for this evil before. He's blamed the Founding Fathers and their political philosophy that guarantees the state wouldn't impose religion on the people. He's blamed Martin Luther. Now he's blaming nominalism, a philosophical theory that perfect, ideal things don't actually exist.
I think Dreher should just go all the way and proclaim that the problem is that too many people think, and their thoughts are too often sacrilegious because they end up questioning the dogmas of the church. Somehow, though, Dreher exempts himself. When he questions the church, as he did when he left Catholicism to become Eastern Orthodox, it wasn't a hateful, uppity, and sinful of him to do that.
However, when Martin Luther questioned the Catholic church, he unleashed an incredible amount of evil. All sorts of people started questioning church dogmas and finding alternative ideas. For example what is more likely to be correct: your priest or your conscience? Is the Church correct that the sun and planets revolve around the Earth? Somehow this questioning is the direct cause of Western civilization becoming decadent and materialistic, and forsaking the guiding principles of a religious life.
Does Dreher actually wish that he could shut down all of that thought, remove the free will of people, and force people to put unquestioning faith in whatever their church says? From what I've read, that is what Dreher would prefer, based on the number of column inches he's devoted to defending dogma ("what Christianity actually is") and decrying the wrongness of guys marrying guys.
Oh, the hubris of using your brain! (... if you might come to a different conclusion from that of the Church).
A moderate problem in Dreher's argument is that Jesus himself is an example of questioning orthodoxy. But Jesus is the Son of God, and inerrant, which no other human is. So it's not illogical if Jesus can question but the rest of us can't. An even bigger problem is that humans are questioning authorities all the time, especially when they aren't thrashed within an inch of their lives for doing so. Maybe European stock is somewhat worse in this behavior than others (how frequently did Indians question their caste system?), but it has been a persistent trait for as long as we have records.
People are going to use their minds, and what they think may change as a consequence. God can't stop it, and neither can Dreher. If it is to be labeled a sin, that tells us that Dreher or the Church or God can't deal with a major aspect of human nature. It also raises the question of whether Dreher believes that people deserve freedom, or whether instead they need the cossetting of an authoritarian system to avoid this worst of sins--thinking the wrong thoughts.
Of course, what is most likely is that Christianity wasn't threatened by Martin Luther, the Reformation, liberty-based government, or a metaphysical theory I hadn't even heard of (and me a philosophy major!). Only Dreher's mistaken and narrow definition of Christianity is threatened and only in his mind.
Extras. Well, the threat's not quite "only in his mind." Traditionalists in some professions have been sued or fined for not working on same-sex weddings. There are also allegations of jobs lost or discriminatory work environments made by traditionalists.
Religious observance and religious identification are also becoming less common. Is that due to people choosing for themselves too? Most likely.