The founders of this country were able to separate their personal religious beliefs from their desire to create a government that is required to respect individual rights. Our government has legal obligation, it does not have moral obligations. Individuals have moral obligations.
With that said, if our society as a whole does not maintain some level of morality, it is very likely that our Constitution will ultimately fail, as it will also fail if our government fails to uphold its legal obligations.
Read the end of it. Year of our Lord is in it.
You are like fish arguing that water is irrelevant to fish food. Leave that water then if you must. The consequences demonstrate how much the fish food is now worth.
Also witness the history that led to the compilation of the constitution. Of all people, Benjamin Franklin recommended prayer when negotiations came to an impasse. He reasoned that if a sparrow cannot fall apart from the will of the Father, how less likely that an empire would rise without His aid. And Franklin wasn’t what we might call a religion monger. The impasse thereupon broke.
Now perhaps a whole history worth of evidence does not move you and that is your choice. But it doesn’t excuse you from the real consequences of not being moved.