Religion plays a big role in keeping society moral. I believe that is why it is the first freedom listed in the BofRights. Morality mattered to our Founders. That's why the left is so hostile to religion. Communities cannot favor a religion, that's unconstitutional, but they can favor a morality. If they can't then where's the freedom?
The argument relies on the premise that morality will only exist by legislation. The absense of morality as a consideration or motivating factor is not immorality, but amorality. When we write our laws, the appropriate question is not "What does the Bible say?" but "What does the Constitution say?". Frequently they will provide the same answer. Sometimes they do not. If we see that as being in error and seek to rectify it by reinterpreting the Constitution to get a different answer, then we might as well discard the Constitution and replace it with the Bible and be done with it. Some things are between you and God, and some are between you and Ceasar, and they can't always be the same.
To the extent that religious freedom is protected, laws are actually less likely to need to address morality, rather than more likely.
I do not advocate "reinterpreting the Constitution to get a different answer" but, in practice, that is what has happened. Unless "we the people" protect and defend the Framers intent, it will get misinterpreted. That is the functional relativity that I observe. It is protected a great deal by the difficulty our Framers placed on changing the Constitution, on replacing judges, and the whole balance of power.
The Framers original intent regarding income tax would have guarded againt socialism better.(see section 9 No. 4) But Congress passed the law, it was declared unconstitutional by the SCOTUS, and so Congress amended the Constitution to fit their will. It can happen. It takes a long time.