Falwell himself once denounced preachers who got involved in governance, though not out of devotion to a secular republic: As a committed segregationist, he decried the work of Martin Luther King Jr, saying, "Preachers are not called to be politicians, but to be soul winners."
Matthew Yglesias picks up on this and offers the following viewpoint:
From the standpoint of religious denominations themselves, though, I suspect that Falwell was offering good pragmatic advice. Religious leaders who involve themselves unduly in political matters become essentially politicians or activist/agitators, two social roles that are much less highly regarded than is the role of religious leader.
I am not sure if that is completely true; Falwell, Robertson, and similar characters seem quite successful and influential to me, and even more so is the Pope, who is clearly a full-time politician as well as religious leader. But there is probably some truth in Matt's observation; in that case, I regard it as a serious problem of our society that religious leaders are more highly regarded than politicians and secular activists. It would be a better world if the opposite attitude prevailed.
Martin Luther King, Jr. was a great man because he fought for a noble cause, with admirable means, and very effectively. That evaluation depends exclusively on secular criteria; and, by the same criteria, Jerry Falwell was an odious man. The fact that they were ministers is only a footnote on their moral biographies, and it should have neither disqualified them from political participation, nor given them any special privileges.
Another commenter on Matt's blog responded that MLK was personally driven by his faith, and that he considered "the moral law or the law of God" as the criterion to distinguish just laws (which we have a moral duty to obey) from unjust ones (which we have a moral responsibility to break). As a principle, this is problematic, because the right-wing religious extremists can claim it, too.
The answer is simple: that principle is bullshit. Faith as a basis of morality is garbage, because faith can justify any kind of moral values, as the comparison of King and Falwell - or, say, Jimmy Carter and Osama bin Laden - easily demonstrates. "If God exists, then everything is allowed" is what Alyosha Karamazov should have said. Insofar as he believed that his sense of Justice came from God, MLK was mistaken. But I don't care what he believed and how he rationalized his values; I care that his actions were right and good. I'd probably deeply disagree with him metaphysically, but so what? Deeds count, not creed.
For the same reason, I don't really care whether televangelists like Falwell believe in the noxious nonsense they preach, and I think that insisting on Falwell having been a deliberate fraud is a weakness in the otherwise very good Christopher Hitchens commentary. It is especially unnecessary for Hitchens to argue that, as he is perfectly willing to say that the faith Falwell preached was evil in itself, even if completely sincere.
The difference between King and Falwell is that the former's deeds were good, and the latter's bad, from a purely secular point of view. That assessment does not depend on their personal religious beliefs. In summary, I don't mind religious leaders participating in politics. What I do mind are bad people participating in politics.