Religion: Slippage from Theology to the "Moral Person"

One of the frequent criticisms made by conservative Christians about "today's Christianity" is that it does not teach the "real" faith. That is, it goes light on the theology of sin and Jesus dying to forgive sins. By getting one's sins forgiven by Jesus one can spend eternity in heaven and avoid eternity in hell. The book I'm reading, Why Religion Went Obsolete, says the faith was about this theology almost entirely until about the 1950's. 

Slowly another version developed. This other version was featured in what is called the "mainline" denominations. Such denominations are some Presbyterian, some Lutheran, some Methodist, Congregationalist, etc. The new version of the faith is one about being a moral person. It's about Christians being "good people." It teaches that Jesus was a moral person. It was this good-person-Jesus that is important instead of Jesus forgiving sin. This new version of Christianity was so popular these denominations grew in the 60's and 70's and became at that time the face of the faith.

This popular version had an Achilles heel, however. It was inevitable realization that there are good people, ones with good morals, that are not Christians. This good morals message was both the reason the faith grew in the decades after World War II but also became the reason these denominations have seen their numbers plummet since 1990. The liberal "moral person" version found its numbers falling more rapidly than the all-sin-all-the-time churches. As the author of the book I'm reading explained, the good moral branch won the battle and then lost the war.

I have read countless versions written by those in the all-sin camp laugh and ridicule the moral-man denominations. In most recent years, however, the all-sin camp has also seen drops in members. The Southern Baptist Convention has lost 10% of its members each year for a decade or so. Some of this is a lower birth rate but it is also loss of members across all ages. 

It seems to me it is going to be harder in the future to plot what is happening at any time in Christian theology. Denominations have official documents explaining their theologies. With more and more churches unaffiliated with denominations theologies will be harder to track. One thing that seems certain, however, is that with each succeeding generation being less religious than the previous one the number of churches will decline.