Michelle Goldberg gives some Jerry Falwell backstory:
The notion of church/state separation, now widely regarded by Republicans as part of a devious war against Christianity, was a widely shared principle. 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."
Classy. The issue of whether or not the old Falwell line on this is correct is an interesting one. As a political matter, I don't really have an objection to religious leaders deciding their moral commitments require them to adopt political commitments as well; it seems like a natural enough thing to do. 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.
At a minimum, the upshot is that religious interventions into political matters are most likely to be effective when they're relatively rare. The rise of people like Ralph Reed who just are political operatives (lobbyists, even!) peddling some kind of "religion" schtick is the logic end of the process -- like a lot of operatives, he has some influence and power, but no meaningful moral or religious authority in the eyes of anyone who doesn't already agree with what he's saying.