Many of the Americans on the Trans World Airlines 727 jet hijacked yesterday after leaving Athens for Rome were Roman Catholic Church members from the Chicago area and Laredo, Tex., who were returning from a tour of the Holy Land and had planned to be home last night.

Instead, they were still being held hostage in the aircraft in the Middle East while parishioners from the three Chicago-area churches whose members had taken the tour held candlelight prayer services and vigils for their safe return. The churches' pastors were captive on the airplane.

One was identified as the Rev. P. William McDonnell of St. Margaret Mary Church in Algonquin, Ill., about 50 miles northwest of Chicago. He helped organize the tour, which began May 31.

The others were identified as the Rev. Thomas J. Dempsey of St. Patrick's Church in St. Charles, Ill., about 40 miles west of Chicago, and the Rev. James McLoughlin of St. Peter's Church in Geneva, Ill., near St. Charles.

McLoughlin's mother, Loretta, was also on the flight but was freed in Beirut.

All crew members were reported still on board, and TWA identified the captain as John L. Testrake, 58, of Richmond, Mo., a veteran of 30 years of flying.

TWA said the crew included first officer Philip G. Maresca of Salt Lake City; flight engineer Benjamin C. Zimmerman from Cascade, Idaho; and flight attendants Uli Derickson from Newton, N.J.; Judy A. Cox of Shawnee Mission, Kan.; Hazel Hesp of Greenwich, Conn.; Elizabeth J. Howes of Wales, and Helen M. Sheahan of Washington, D.C.

Sheahan, 42, who has worked for TWA for about 20 years, left home Thursday for an overseas assignment expected to last several days. She was looking forward to the trip, on which the crew included colleagues with whom she particularly liked to work, according to her husband, Dr. Stojmil Petkov, a physician.

Now, he said, "one doesn't know what to do. One can only hope."

Thirty-seven of the 145 passengers who boarded the ill-fated flight in Athens were from the three Illinois churches.

More than 600 parishioners gathered solemnly at St. Margaret Mary Church last night to attend a mass for the hostages, who included 18 parishioners, some in their 60s.

With hugs and tears, the group sought to support hostages' relatives in attendance. As darkness fell on the 2-year-old church, robed children strummed folk guitars and sang of hope and love.

Eva Hill, sister of hostage Simon Grossmayer of Algonquin, attended the mass and said she and her husband had planned to take the tour but changed their minds.

She said her brother's children had talked to their mother, Elaine, who was among about 40 persons released from the hijacked plane. "I felt good that Elaine is free, but I worry, where is my brother? What can you do but say your prayers for them?"

Pat Ryan, 17, said of his grandmother, released hostage Neta Slotowski, 67, "She's deeply religious. That's why she wanted to go. I told her not to go or to bring a gun with her but she went anyway."

Connie Ryan, Slotowski's daughter, said her mother "has profound trust in the Lord. She has a very cool head."

Connie Ryan said she talked to her mother, who reported that the hijackers had forced the passengers to travel from Athens to Beirut with heads bowed on seat-back tables and hands locked behind their heads.

Parishioners said McDonnell and several lay activists and nuns had built the parish from 400 to 1,400 families during his six years as pastor. They said they have always felt particularly close to each other, and some said they think of themselves as "part of a family."

The church plans to celebrate another mass for the hostages today. "We'll stay here until this thing is resolved," said associate pastor the Rev. Robert Garrity, 30, who celebrated last night's mass.

McLoughlin's mother "wanted to know if we were all praying, and I told her we were," said Lois Bushman, a pastoral associate at St. Peter's who had telephoned Larnaca, Cyprus, where hostages freed in Beirut were flown.

"I told her we were starting a vigil, that we loved her and were glad she was safe and that we loved him and would keep on praying for him," Bushman said.

She said that Loretta McLoughlin was weeping during their conversation and that they talked for only "a minute or two."

The tour left Chicago and stopped in New York, Paris, Tel Aviv and Athens, with some members leaving Athens for an optional cruise of Greek islands.

Seven family members from Laredo also had toured the Holy Land. They were Vincente Garza, 53; his wife, Irma, and their daughter, son-in-law and three granddaughters. Irma Garza, her daughter and two of her granddaughters were released in Beirut.

Garza owns several department and grocery stores in Laredo, according to his brother, Jorge.

"They usually get together once a year and go on vacation," Jorge Garza said. "This year they decided to go to the Holy Land and then to Rome." He said he had talked to Irma Garza after she was released. "She was crying and doesn't know what to expect," he said. "They took all their money. They're pretty shaken up right now."