ST. PETER'S Episcopal Church stands diagonally across from the Cambridge City Hall. The building is square and redbricked, squeezed in between storefronts and a narrow alleyway of a street. It is an old church--founded in 1842, four years before Cambridge became a city.

Rev. Ernest D. Thompson has been rector at St. Peter's for 22 years. In that time the composition of the parish has changed. Attitudes toward religion have changed, both in the parish and in churches in general. One of the biggest problems facing St. Peter's is the changing attitude towards the function of churches and of organized religion. In St. Peter's it is a battle between the ancients and the moderns, with the rector leading the moderns.

When St. Peter's was founded, the arrangement of service reflected the social order of the day. There were two sets of services: a morning service for the employers, an evening one for their employees. "About 80 per cent of the members originally came from the Maritime Provinces," Thompson said. "They were basically well-established, conservative people."

"When I first came here, there were three services, and all of them were packed," the rector explained. "We had two choirs, a women's auxiliary, a women's guild, a men's club... Nowadays you could play ping pong or hockey in the aisles at the 10 o' clock service." On a cloudy Sunday in early March, about 60 people were scattered throughout the church which was built to hold 300. Sunny days do not draw too many more people.

THOMPSON maintains that this is partly because St. Peter's is a city church. Episcopal churches do not draw their members from any strict geographical boundries, and city churches lose their members to suburban churches. But part of the reason that this once-bustling church now echoes with the silence of disuse may be because the functions of churches are being taken up elsewhere.

Thompson has two churches, he says: his Sunday parishioners and his weekday parishioners. For various reasons, the Sunday congregation -- made up of the regular church members -- is declining in number. The weekday parishioners on the other hand, are coming to St. Peters in droves. With dwindling membership, money to run a church must come from outside sources. During the week and in the evenings, St. Peter's parish halls are rented to outside groups which meet there. The Greater Boston Peace Action Coalition, a yoga class, a ballet class, and a Sufi group are currently using the church as their meeting place. "St. Peter's is well known by all kinds of groups and organizations," Thompson said. "They call up all the time to rent space, but our walls are bulging during the week."

Thompson said that in some ways he felt that the weekday congregation was more devout and more genuinely religious than the Sunday congregation. One of the reasons, he said, might be that Sunday worship in an organized church has lost some of the mystique of religion. "Familiarity breeds contempt," he explained.

A certain "contempt" for organized religion on the part of younger people, Thompson said, may be decreasing the church's Sunday population. "Most of our old members are dying, and there are not many younger ones to replace them," Thompson said. "The younger people say they're not interested in the double standards and hypocrisy of the organized church. I think they may be right."

The "double standards" in religion that Thompson talks about, are for him, the fact that people try to isolate their religion in their churches. "The people are dedicated to their church -- building," he said.

Thompson said he feels very strongly that the church is far more than social gatherings and that religion means more than praying. "People who come to church regularly every Sunday but then just forget it the rest of the week are trying to make a sucker out of God. There is a big difference between being 'religious' and being 'good,'" he said.

Thompson preaches on Sundays in the regular cassock of a high church minister, with one exception. On one side of his robe is a large embroidered peace symbol: the circle with the "chicken track" inside. For many years he has been using his Sunday sermons to preach strong antiwar messages. This, combined with his insistence on preaching involvement in other current social issues, has been an area of contention between him and some of his parishioners.

CERGYMEN are turning over faster than parishioners--some of them because they can't take it. The clergy today are much more liberal than their parishioners," Thompson said. "People only want a nice warm feeling in the pit of their stomach from a sermon. You can see them close their minds when you try to convince them of anything different. The older members of the church think the church belongs to them and they just want to be left alone. I've been beating my head against the wall trying to get them to change that idea."

However, the people of St. Peter's seem to agree with many of the things Thompson says. After the service on Sundays there is a coffee hour and people meet to discuss the sermon and the week's happenings. It is a friendly gathering. Most of the people have known each other since childhood. They are eager to make a stranger welcome in their church, and equally eager to talk about the problems of the church. For them, as well as for their rector, some of the problems stem from "the older members of the parish."

THE YOUNG members have to make the older ones change," Phillip Green, a member of St. Peter's said over coffee. "We have to learn to give a little more. St Peter's hasn't been a giving church."

"That's because at first there was the attitude, 'This is our church,' "Ruth Marshall said. "I think that attitude is changing, though," she added.