Why do yanks forget this one when they talk about religious establishment???

Article Six of the United States Constitution:

no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.

Why is this in the Constitution if this is a Christian nation?

Also, why is this:

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;

in the the Constitution as well if this is a Christian nation?

I think that is pretty clear that the US is supposed to be religiously neutral. How can there be people like these?

The US is supposed to be secular. Got that?