If invocations are done, they have to be, according to (a 1983) Supreme Court decision, such that they do not advance any particular faith or belief. The reason being, you try and make as many people included as possible so that nobody feels that they're being left out of any particular prayer.
Quotes about invocations (2 Quotes)
I thought and pondered - vainly. I felt that blank incapability of invention which is the greatest misery of authorship, when dull Nothing replies to our anxious invocations.