Whores perform the same function as priests, but far more thoroughly.
[Robert Heinlein, Time Enough for Love (1973).]

Theology is never any help; it is searching in a dark cellar at midnight for a black cat that isn't there.
[Robert A. Heinlein, JOB: A Comedy of Justice, (1984).]

Anyone who can worship a trinity and insist that his religion is a monotheism can believe anything... just give him time to rationalize it.
[Robert A. Heinlein, JOB: A Comedy of Justice, (1984).]

The Ten Commandments are for lame brains. The first five are solely for the benefit of the priests and the powers that be; the second five are half truths, neither complete nor adequate.
[Robert Heinlein, Ira Johnson in To Sail Beyond the Sunset.]

It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so, and will follow it by suppressing opposition, subverting all education to seize early the minds of the young, and by killing, locking up, or driving underground all heretics.
[Robert A. Heinlein, Postscript to Revolt in 2100.]