Something many people do not realize, perhaps because they have heard so many arguments to the contrary, is that the words “separation of church and state” do not appear anywhere in the U.S. Constitution.
The doctrine of separation of church and state was derived from a letter written by Thomas Jefferson, to assure members of a Baptist community in Connecticut that they would always be free to practice their religion. The assurance was that a “wall of separation between church and state” would preclude the state of Connecticut from forming or enforcing a state religion.
Today, that doctrine has been erroneously expanded to encompass some imagined concept of freedom from religion. Unfortunately, doing so contradicts the second clause of the First Amendment, the free exercise clause. Aside from that fact, it is the first clause of the Amendment that usually gets the most attention. This clause has also been erroneously expanded as a claim that the mere mention of God or the mere influence of religious principles in forming an opinion is somehow unconstitutional.
The truth is that Congress is not allowed to make laws that establish, prefer, or promote any particular religion. That does not mean all things religious are completely forbidden. So, when the argument arises that separation of church and state means that marriage cannot be addressed or defined by any level of government, that argument is without merit.
The argument is often posed in reference to alternative definitions for marriage. The argument is incorrect on at least three counts. First, it is not one particular religion that rejects this idea. Second, there are people who are not religious who also disagree. Third, even if Congress made a law denying gay marriage that is not the same as making a law that enforces one particular religion.
The Constitution is supposed to be interpreted in accord with the common understanding of the language used, at the time it was written. Perhaps a better explanation can be found by breaking down the establishment clause:
“Congress shall make no law respecting an establishment of religion…” simply means that Congress can make no law in respect to creating (establishing) a specific religion or in respect to one specific religion above others. It does not mean that Congress cannot show respect for religions (or God) or that government and religion must be completely separated in all aspects.