I wrote about this in an op-ed in The Christian Science Monitor at the time:

At a rally on the courthouse steps, James Dobson said:

"I checked yesterday with my research team," Dr. Dobson announced. "There are only two references to religion in the Constitution." The first, from the preamble, he said, refers to securing "the blessings of liberty," which, he asserted, "came from God" (although there is nothing in the document to support that view.) The other was the First Amendment's establishment clause that, he said, "has given such occasion for mischief by the Supreme Court."

However, Dobson's researchers missed - or ignored - Article Six of the Constitution. That's the one barring religious tests for public office and set in motion disestablishment of the Christian churches that had served as arbiters of colonial citizenship and government for 150 years.

Mainstream historian Gary Wills writes that the framers' major innovation was "disestablishment."  

In a November 2006 interview with Larry King, Dobson indulged in some well debunked revisionist history. I wrote about this at the time. Dobson claimed that there is no such thing as separation of church and state.  The historical record clearly shows that while it is true that the phrase separation of church and state is not found in the constitution or the first Amendment, the phrase was in wide use among leading thinkers at the time, and that Jefferson's letter to the Danbury Baptists was an effort by Jefferson to explain his understanding of the meaning of the First Amendment. That is part of the reason why the Supreme Court has used the phrase since at least 1878 to explain the meaning of the First Amendment, and why it has become a standard part of our understanding of the meaning of the Constitution.

KING: But we have a separation of church and state.

    DOBSON: Beg your pardon?

    KING: We have a separation of church and state.

    DOBSON: Who says?

    KING: You don't believe in separation of church and state?

    DOBSON: Not the way you mean it. The separation of church and state is not in the Constitution. No, it's not. That is not in the Constitution. That was...

    KING: It's in the Bill of Rights.

    DOBSON: It's not in the Bill of Rights. It's not anywhere in a foundational document. The only place where the so-called "wall of separation" was mentioned was in a letter written by Jefferson to a friend. That's the only place. It has been picked up and made to be something it was never intended to be.

    What it has become is that the government is protected from the church, instead of the other way around, which is that church was designed to be protected from the government.

    KING: I'm going to check my history.

If King did his homework, he would find whatever any honest researcher finds in response such claims about church state separation. Last year I published a long essay in The Public Eye magazine, titled History is Powerful: Why the Christian Right Distorts History and Why it Matters, and in it, I quoted a classic scholarly debunking by Brent Walker of Dobson's claim, as articulated by Christian historical revisionist, David Barton:  

Of course, neither the words "church state separation" nor "wall of separation" appear in the Constitution. That does not mean Barton's position is correct. The Constitution does not specifically mention "separation of powers" or "the right to a fair trial" either, but who would deny the Constitutional status of those concepts? "Church-state separation" is a metaphor for what certainly was and is the spirit of the First Amendment's religion clauses - government is to be neutral toward religion to the end of ensuring religious liberty.

Barton mentions church-state separation as flowing from Thomas Jefferson's 1802 letter to the Danbury Connecticut Baptist Association. He asserts that later in the letter Jefferson made it clear that he wanted only a "one directional wall" to prevent the government from harming religion, not to prevent religion from capturing the government.

A reading of the entire letter belies any suggestion that Thomas Jefferson thought it was "one directional." There is absolutely nothing in the letter even to hint that that is the case.

Dobson and Obama can disagree all they want about the proper interpretation of the Bible. But Barack Obama taught constitutional law at the University of Chicago for twelve years, while James Dobson has a long record of issuing utterly spurious declarations about the constitution with complete conviction.