There’s sort of a two-pronged answer to the OP’s question.
One comes from Alexis de Tocqueville, and his famous observation-- “America is great because she is good. If America ceases to be good, America will cease to be great.”
Today, American has, in large part, ceased to be good. (Not that it ever was, completely, but there was much more reverence for the things of God in the past.) We have many people who deny that Christianity played any role in the founding of the nation–even though most of the Founders were religious people in some form or another. Even Benjamin Franklin proposed prayer during the Constitutional Convention. There would be howls of outrage and loud cries of “Separation of church and state!” if such a thing happened today. We have many people who celebrate and promote sexual perversion in the name of “equality.” We have many people who are actively trying to tear down traditional standards of morality and decorum in the name of “progress”–witness all the high school girls who have made the news for fighting against dress codes. A school just this week made the news for not letting a boy who had long hair attend school–and many people are outraged over that, when they should be outraged at his parents.
Going along with that, many people today think of the 1950s as being the last period when America was “good.” Now certainly, there were many problems; racism was very widespread, even among people who (falsely) claimed to be Christians. But there were standards of decorum, and standards of at least outward morality, that have been almost totally abandoned in the last 40 years. Premarital sex was still considered shameful, instead of being celebrated in the name of “love.” Most people dressed reasonably modestly back then, instead of thinking that it would be ok to go half naked in public. Women wore skirts and dresses, instead of cross-dressing. Divorce was scandalous. Most people went to church, instead of treating Sunday as merely another day of the week. Even though many people were not actually Christians, there was a degree of religious feeling and religious knowledge that is totally lacking today.