The Soviet Union Thursday abruptly canceled one of President Reagan's meetings with General Secretary Mikhail S. Gorbachev and questioned the composition of a religious group scheduled to meet with Reagan in Moscow.

The session, called off because of what the Soviets cited as "some internal function," was one of five that Reagan had scheduled over four days with his Gorbachev.Overall, the president still will spend six hours with the Soviet leader because the remaining sessions will be expanded, said White House spokesman Marlin Fitzwater.

"I regard it as routine," Fitzwater said, as Reagan spent the day in seclusion at a government guesthouse making preparations for his talks in Moscow.

The summit encountered another glitch when word reached the presidential party taking a rest stop in Finland that the Soviets had questioned the participation of one of the religious groups due to meet with Reagan at a monastery on Monday.

"The meeting itself is still on as far as I know," Fitzwater said. He said he did not know which religious group might be excluded.

Reagan also plans to meet separately with a group of Jewish "refuseniks" who had been unable to obtain exit permits for Israel. Fitzwater said he was unable to verify a report that two of them had been intercepted by Soviet authorities on their way to Moscow from Leningrad.

On the official level, the Soviets have shown no concern over the meeting planned between Reagan and Soviet refuseniks. At a press conference to discuss the forthcoming summit, U.S. affairs expert Georgy A. Arbatov and Communist Party Central Committee official Nikolai Shishlin both stated that they considered Reagan's meeting partners his own business.

Reagan's first visit to the country he once denounced as "the evil empire" was already clouded by failure of a U.S. effort to oust Gen. Manuel Antonio Noriega, the military ruler of Panama.

After the summit, Gorbachev will preside at the first full-fledged Communist Party conference since 1941, but there was no immediate word on whether the cancellation of the second of Reagan-Gorbachev sessions on Monday had any connection with the party meetings.

For the Soviets, the party conference has tended to overshadow Reagan's fourth summit meeting with Gorbachev, because it will serve as a test of the Soviet leader's economic reform policies and his three years at the Kremlin helm.

At the same time, the summit itself has lost any expectation of major results with the failure of U.S. and Soviet negotiators to complete a treaty to sharply reduce long-range nuclear weapons. Reagan and Gorbachev had hoped to make such an accord the centerpiece of the 77year-old president's first trip to Moscow.

In an interview Tuesday with a group of West European jouralists, Reagan left open the possibility of a fifth summit with Gorbachev before he leaves office in January - if negotiators can complete the treaty to reduce strategic nuclear weapons.

"I won't rule it out," Reagan said in a text released here Thursday. "I won't say it's impossible . . . I would hope that we can iron out the still undecided points before I leave."