In the first amendment Congress not establishing a religion means they shouldn't make a federal state church (sponsored by the federal government), or force anyone to be a religion, it does not mean those governing a largely Christian people should not represent that morality and world view in their governance, or laws. There used to be anti-adultery laws, and that was obviously because of Christian morality, but it didn't require practicing Christianity to obey them.
The founders weren't deists. A deist cannot say "God governs in the affairs of men," as Benjamin Franklin said at the Constitutional Convention when he recommended they begin with prayer and have clergy officiate it. While they didn't want denominational persecution by law, the idea that they banned Christian morality guiding how the nation was governed is also false. As near as I can tell, the so called "Christian nationalists" that the left fear, don't want anything different than the founders wanted. I don't think state mandated Catholicism or Protestantism or even state mandated belief in Christianity is part of that.
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