It isn't your typical ad. It hangs on the wall of the Metro subway car along with advertisements for cigarettes, money market funds, glasses and an airline.

But this one is marketing the Catholic Church. The appeal is aimed at Catholics who have dropped out of the church: "We want you to be an active part of us again," it says.

The ad is one of 175 placed in subway cars this month by the Roman Catholic Diocese of Arlington.

This week, a two-week broadcasting of radio commercials also began in the month-long campaign.

"The church has to be in the marketplace, just like everyone else," said Ellen McCloskey, director of communications for the diocese and originator of the campaign.

The U.S. Catholic Conference called five years ago for evangelization of Catholics who had left the church and other "unchurched" Americans.

Four years ago, the dioceses of Philadelphia, Camden and Trenton sponsored a four-month-long radio ad campaign. The campaign, costing $72,000, used testimonials on the church from celebrities ranging from Phillies baseball center fielder Gary Maddox to folk singer Arlo Guthrie ("I think God asked me to be a Catholic," Guthrie's testimonial said).

A half-dozen other dioceses have launched return-to-church campaigns in recent months or are now planning them.

Most include commercial radio advertising, said the Rev. Alvin Illig, director of the Paulist National Catholic Evangelization Association, a Washington-based center promoting evangelization.

Last year, about 4,000 people in all were trained at three evangelization conferences sponsored by the association.

About half of the country's Catholics no longer attend mass regularly, and 26 percent of baptized Catholics over 18 years old are inactive in the church, according to Gallup polls.

The reasons for not attending church vary, said Illig. The most common reason stated was related to marriage: people had joined their spouses' church or married a nonchurchgoer, he said. The dropouts also included divorced Catholics without church annulments of their first marriage who had remarried outside of the church, he said.

Other reasons given included "poor preaching," a move to a new town, or too little change in the church in areas such as birth control, women's ordination, or democratization of the church, Illig said.

Still other Catholics think there has been too much change, or they dropped out because of a personal or family quarrel with a priest, nun or other parishioner, he said.

The size of Catholic parishes is a factor. The average Catholic church has 2,500 parishioners, compared to the average Protestant church membership of under 200, Illig said.

"They say, 'They don't know whether I come or not, they don't miss me.' It becomes impersonal," he said. But there aren't enough priests to have smaller parishes, he said.

While the number of Catholics in the United States increased from 48 million in 1970 to 51 million in 1981, the last year tallied, the number of priests decreased from 58,161 to 58,085.

The Arlington diocese's ads include appeals to people who left the church in anger. "Dear Beth," one of the radio ads begins, "we want you to know that we love you, you're still special to us. You know, it takes two to understand and forgive."

Others are less specific. "Since I've returned to church , I've found something to believe in and a community that cares for others," says one.

The campaign will cost about $5,500, most of that for the radio time and subway space, McCloskey said. The money will come from a communications collection taken in the parishes last June, she said.