The Framers were very straightforward about everything in the Constitution because they wanted it to be clear for the rest of the country's existence. Read the other 9 Amendments of the Bill of Rights -- all short, concise, and clear with no room for interpretation. Why do you believe the 1st is any different?
Jefferson and Lincoln?
Jefferson: "I have sworn upon the altar of God eternal hostility against every form of tyranny over the mind of man."
Have you ever read Lincoln's Second Inagural? 702 words -- it referrences God 19 times and cites the Bible twice. Only 702 words.
I'm not so sure that the rest of the Constitution is all that clear. Take the second amendment, for example. Does it really mean that anyone can have any kind of arms that they want? Remember, arms means more that just guns. The Supreme Court exists to interpret the Constitution. If it were so clear, the SCOTUS would have no reason to exist.