At some point during this long summer, the "wall of separation" between church and state became a battleground. Not the least well-armed of the contenders were the Catholic hierarchy assembled under the anti-abortion banner.

First we had New York Archbishop John J. O'Connor saying that he didn't "see how a Catholic in good conscience can vote for a candidate who explicitly supports abortion." In tandem, Bishop James W. Malone, the president of the National Conference of Catholic Bishops, said that Catholics couldn't draw a line between "personal morality and public policy." They were both taking special aim at Catholic politicians of the Cuomo-Ferraro stripe who are "personally opposed to abortion but . . . "

Then last week, 18 New England bishops signed a statement that put two issues at the top of their moral/political agenda. These things were to be considered by voters above all others: abortion and nuclear war.

To the amateur observer this might have suggested that the Catholic church is splitting its ticket, since Reagan is seen as more sensitive to the unborn, while Mondale is regarded as more sensitive to the born. But the bishops said that the abortion issue comes first because: "While nuclear holocaust is a future possibility, the holocaust of abortion is a present reality." Presumably they will allot nuclear war prime time during the nuclear winter.

Frankly, I am not one of those who believe that every clerical collar should come with a muzzle. Clergy have every right to speak on moral issues. They even have the right to endorse legislation and candidates. But when religious leaders start to talk like lobbyists and politicians, the public has to judge them as they would any other public-interest group, like The Sierra Club or the National Rifle Association or the National Abortion Rights Action League.

At some point, we are no longer getting a sermon but a mass mailing. We are no longer a respectful congregation but a skeptical constituency. It's altogether appropriate to deal with the lobby this way, to check their facts, to ask whom they represent and whether the public policy they support is the best way to deal with the issue.

The bishops imply, for example, that the "facts" on which they rest their political case against abortion -- that the fetus is a person and that abortion is therefore murder -- are universally accepted within the Catholic church. But Catholic theologians are still arguing about when the fetus becomes a person. Before the 18th century, the Church refused to baptize aborted fetuses because they were not viewed as human.

The question of whom exactly the bishops represent -- beyond other bishops -- is also a bit murky. They do not speak for 53 million American Catholics. Catholics share the same conflicts and attitudes toward abortion as the rest of Americans. Less than 20 percent of them agree with the bishops' support of a ban on all abortion. Indeed, at least in Massachusetts, Catholic women have abortions at the same rate as women of other religions.

One of the things that may have prompted the hierarchy into electioneering is the public image of pro- choice Catholic politicians, especially Geraldine Ferraro. As Frances Kissling of Catholics for a Free Choice says, "Ferraro is such a visible sign of the Church's inability to control the Catholic people. Here is a woman on the front page every day who doesn't agree with them and goes about the business of being a Catholic." Just last weekend, Archbishop O'Connor complained that "she has given the world to understand that Catholic teaching is divided on the subject of abortion."

But if the bishops prefer to think of themselves as representing God or His will, we get into even deeper religious/political trenches. There has been no divine revelation on, say, the Hyde Amendment. It is perfectly legitimate for any citizen, including Catholics with deep qualms about the morality of abortion, to argue over "pro-life" legislation. Would re-criminalizing abortion mean less loss of fetal "life" or more loss of female life?

Furthermore, the Church has an internal quandary about its own responsibility for unwanted pregnancies. On the very day that Boston Archbishop Bernard J. Law and the other New England bishops were taking a political stand against abortion, the pope proclaimed that even "natural family planning," the rhythm method, the one form of Church-approved birth control, was also questionable. In any great civilian war, politicians like to claim that God is on their side. If God is unavailable, a volunteer clergyman makes a formidable recruiter for the party. Archbishop Law protests, "I don't want to be a political boss." But if a clergyman talks like a political boss and walks like a political boss, he must be judged like a political boss.