Any candidate who arrives at a policy position because of his religious faith should be free to express that connection so voters can make an informed decision. Because I think his theology or scriptural interpretation is poor does not give me the right to tell him to shut up. I can, in response, tell Rick Santorum that contraception is not evil, tell Newt Gingrich that Jesus probably would support marriage equality, and tell Mitt Romney that our care for "the least of these," both as individuals and as a society, is probably a more Biblical measure of our faithfulness than our concerns for the middle class.
Of course, as believers are always glad to remind us, the phrase "separation of church and state" do not appear in the U.S. Constitution, but were coined by Thomas Jefferson, who had no role in writing that document, some years later. The charter itself restrains Congress from "prohibiting the free exercise" of religion or to establish a state religion. A state religion existed, as they did in several of the original states, when people or property were taxed to support a particular church. For Jefferson, the "wall of separation" kept the state from persecuting religious minorities or dissident individuals." Neither Madison nor Jefferson would have seen the criminalization of any particular "sin" as establishing a religion -- though they may well have laughed a candidate for office who claimed to have detected a Satanic conspiracy against the country out of the capital. The point of this is to question whether Knowles-Tuell's distinction is actually useful. My hunch is that he thinks it would be unconstitutional for Santorum to sign a law against contraception if he cites biblical authority, or for Gingrich to sign one forbidding marriage equality on similar grounds. But the Constitution need not forbid these things unless we advance a new meaning of the word "establishment" that equates it with our modern understanding of theocracy as refined by Americans' aversion to sharia law. Even then, I'm not sure how much homophobes and anti-contraceptionists would be constrained, since the burden of proof would still be on litigants to prove that any law against contraception or marriage equality was based exclusively on religion. I'm pretty certain that there are homophobic or anti-abortion atheists out there, and in any event homophobes in particular have long known the wisdom of appealing across cultural lines to any numbers of "traditions" that hate gays. Just because may ancient faiths share oppressive "moral" traditions, the notion that modern laws based on those traditions amount to the establishment of a particular religion -- and in the Founders' time it was not so much Christianity itself but one of its denominations that usually was established -- probably wouldn't pass muster. All this means is that the only safeguard against believers "imposing their values" in any way except establishing their church or forbidding others is political agitation and electoral success. For that purpose, Knowles-Tuell has the right idea. We should know the extent to which politicians base their positions on faith or myth so we can challenge them to offer any basis for those positions beyond "God says so." Then, since they'll already have spoken their piece, we'd be entirely within our rights to tell them to shut up. We wouldn't be able to force them to shut up, except by beating them at the polls so that they can't speak for us.