I would say they've been proved right by what has happened recently. Thankfully, it's not too late yet for America, and hopefully things change in 2024. But the situation is dire.
Also, if I recall correctly, that statement of Pres. John Adams mentioned above was in response to the anti-Christian French Revolution, which was agnostic. I wonder, if the American Founding Fathers had seen the Atheistic Communist Revolution, which as mentioned was far worse, and led to some 20 MN Christian victims, what further steps they would have taken.
But even greater imo is the ongoing Abortion Holocaust. We have millions of unborn children being killed every year. Atheism/Agnosticism/Communism/Leftism is unable to prevent it and even complicit in causing it to occur. A return to State Christianity is necessary and the only way to safeguard the future and ensure "Never Again" for this Abortion Holocaust.
God Bless.