opening with prayer is an official act of the entire legislative body.
The official acts of congress are known as "Acts of Congress". Can you identify which, if any, act of congress has to do with opening with prayer?
As far as I know, this is simply part of their working procedures, and not in any way an official act.
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I'll make my position clear. I would much prefer that there were no chaplain, and that there were no opening prayer. The congressmen should offer their prayers in private. The employment of a chaplain and the opening with prayer is an arrogant political act, and has little to do with the religious beliefs of the participants.