He stood up at the pulpit as the chanting died down, a big broad-shouldered man with sweat on his forehead an his shirt open at the collar. He was from Jersey City, N.J., a postal clerk by trade, and like most of the crowd that filled All Souls' Church here yesterday afternoon, he was unemployed.

"My name is Mac," he said.

James Robert McCloskey is his formal name, but yesterday he was just Mac the organizer, New Jersey chapter leader of the Unemployed Workers Organizing Committee. The white banner above him said, "No Cuts in Unemployment Benefits," The black banner below him said, "Unity in Action." And every time he gripped the pulpit and paused for effect the crowd roared, "No Cuts! No Way!"

The rally was called to protest a bill now before Congress that rally sponsors said would cut back the federal program that now extends unemployment benefits from 39 to 65 weeks.

According to Capitol Hill sources the program, which was enacted in 1974, is due to expire March 31.

A House Ways and Means Committee source said that the new bill would prevent the emergency program from expiring completely, but would provide a maximum of 52 weeks of benefits.

In addition, protestors criticized what they said is a provision in the bill that could require those receiving benefits beyond 39 weeks to accept any job paying the minimum wage or above. Before such a provision could be invoked an individual's prospects for finding work in his customary occupation would have to be judged "poor."

The hill, which has been approved by a subcommittee of the House Ways and Means Committee is expected to win the approval of the full panel on Tuesday.

"Slave labor," the protesters called the bill yesterday. They said it would force workers into low-paying jobs and depress other workers' wages. Johnny Rogers, a bindery worker from Chicago, said he had been laid off from a $5.35 per hour job when American Can Co. closed up and that he had turned down a couple of bindery jobs. He had 10 years' experience, he said and they had offered him $2.75 per hour.

"That's ridiculous," Rogers said. He had to think for a while to describe his reaction to that salary offer without using profanities. Rogers said his wife also was laid off from her job at American Can, and that they had five children. He said he survives on part-time work and odd jobs, but that he will not take a bindery job at half his former pay.

Unemployed workers had pulled in from all over the East Coast and Midwest yesterday, most of them drawn by the 33 UWOC chapters located mostly in industrial cities. They were steelworkers, seamstresses, carpenters, truck drivers. Some had never attended a demonstration before.

Shirley Pall had come down from Lawrence, Mass., where she had been laid off from her job sewing sports clothes in a factory. She sat quietly in one of the pews, looking a little alarmed as the cries of "No cuts! No way!" swelled up and echoed through the church. "I've never been in anything like this before," she said.

David Zanckman, a 56-year-old unemployed packer from Philadelphia, also was quiet during the chanting. He held his hat on his knees and said he had been laid off in December, 1975. "My benefits are about running out now," he said softly. "From there I don't know what I'm going to do. They take younger fellows. They don't want to pay you nothing."

There was about 800 of them in all, according to police counts. They railed against the bill and unemployment in general. Mac clenched his fist from the pulpit and called for support for "the over-all war for the working class against those who live like kings . . . and are trying to crush us."

Then they filed out into the sunshine and marched out Columbia Road, down 18th Street, and toward the White House. They had a letter for President Carter. "Why do you represent the monied interests of the employing class while claiming to speak in our behalf?" the letter asked.