In Minneapolis, an Episcopal church project is offering posters picturing the mushroom cloud and asking, in giant type: "Will Man Destroy in Six Minutes What It Took God Six Days to Create?"

The Episcopal bishops asked that question last fall in a pastoral letter. Now the Roman Catholic bishops are preparing a similar pastoral letter.

With all respect, my own churlish suspicion is that the opinion of bishops on the fine points of nuclear deterrence is approximately as valuable as the opinion of generals on the fine points of transubstantiation.

But the church, for its soul's sake, must sometimes put on the cloak of Nathan, the prophet who chastised King David, the most famous warrior of an earlier day. It cannot stand indifferent to great issues of war and peace, however complex they may become.

About one development, the writers of the draft of the Catholic bishops' letter are right. A significant shift in U.S. deterrence policy is afoot -- a shift from "assured destruction," which assumed that nuclear war could be made too threatening to contemplate, to "war fighting," which contemplates and looks beyond the possible failure of deterrence.

The older deterrence policy was designed to inflict unacceptable damage on cities, populations and industries. Now that nuclear weapons are capable of greater speed and accuracy, targeting can be shifted--away from people and cities to weapons, military installations and so-called "command and control" centers.

If you take as a starting point the sanctity of human life (especially of noncombatants), the new deterrence is an improvement. The emphasis is shifted from "civilian" to "military" targets, when they are distinguishable. But the same shift also, in a measure, makes nuclear war more possible by making it, in a military sense, more "rational."

That is the moral paradox of the new deterrence. And here, the bishops' pastoral letter seems contradictory. It condemns both the use of nuclear weapons "for the purpose of destroying population centers or other predominantly civilian targets" and "the willingness to foster strategic planning which seeks a nuclear-war fighting capability."

Unfortunately, the only strategy that obeys both strictures is no strategy at all.

And there is a larger problem in attempting to apply Christian values to political conflict.

Of old, the church's peacemaking role assumed a universal Christendom. Man's sinful nature ensured conflict; but the conflict did not arise (except in the age of the Crusades) from a war by one side upon the foundations of the faith itself. But such a war is one aspect of the East-West confrontation today.

Responsible churchmen, especially bishops, have an obligation not to overlook the asymmetry of values between the West and the Soviet Union. Can the church prudently teach compunctions that may disarm the faithful, but not those who attack and persecute the faith? And if it does, would that make war more, or less, likely?

The bishops do face one difficulty foursquare. In advocating a U.S. renunciation of "first use" of nuclear weapons, they candidly observe that such a renunciation involves "a willingness to pay higher costs to develop conventional forces." Indeed it does. And beyond that cost, it could entail a renewed draft, without which deterrence by conventional forces is unlikely to be credible.

When bishops leap into the intricate maze of nuclear strategy they must face the hard questions that vex good minds and astute consciences other than their own. A miter does not confer a license to simplify.