Why Was Religious Indoctrination So Successful?

Once up a time, not so long ago, in the early to mid-20th century, organized religion (mostly Christianity and Judaism in the U.S.) were extraordinarily successful at getting almost everyone to describe themselves as an adherent of it, to claim belief in God, and to hold onto those childhood beliefs into adulthood and old age.

Pretty much everyone in my parents generation and older was a believer. A huge swath of people in my generation are not, and the nation is become less religious almost every year.

Europe underwent a similar transformation about a generation earlier.

What was organized religion doing back then that made it so successful and why did this collapse in a just a generation or two?

The question often gets asked from the other perspective. Why are people these days so non-religious. But, from our perspective, the real question is not that, but why it used to be so successful. It is something that people in my generation, which is when the transition really took off ought to be uniquely qualified to answer.

I won't answer the question in this post, but wanted to ask it while I am thinking about it.