A D.C. policeman, testifying yesterday as the first defense witness in the trial of nine members of the Black Hebrew religious sect on federal racketeering charges, said it was he who sublet to the group a Southeast Washington apartment that was later wiretapped and searched by the FBI as part of its investigation of the sect.

The officer, who said he is a Black Hebrew "brother," also testified that he flew to Liberia in December 1984 on an airline ticket given him by defendant Cordell DeBardelaben. Prosecutors have alleged that the ticket was among a number stolen from New York Air.

Officer Herman J. Keels, 38, said his name was on the ticket, which required him to change planes in Brussels on the way to Monrovia from New York, but he testified later that he never examined it.

"I would not travel on anything illegal," Keels said.

"Have you ever been asked by [the sect] to do something illegal?" asked Assistant U.S. Attorney J. Michael Hannon.

"No," Keels said. He later acknowledged that J.C. Vortis, another of the defendants and the head of the Black Hebrews in Washington, twice asked him about obtaining guns -- a .25-caliber automatic and a 9-mm weapon.

Keels said Vortis told him that the automatic would be used by a bodyguard for Nation of Islam minister Louis C. Farrakhan. Keels said he did not consider the request illegal.

Keels also testified that he had extracted information from the Washington Area Law Enforcement System, a computerized data base, at the request of another Black Hebrew who is not charged in the case. Keels contended, however, that any individual could request police to retrieve material from the system.

A 16-year veteran of the police department who works as a plainclothes officer in the prostitution squad, Keels said he joined the sect in 1982 and had given it more than $10,000 since then. He has not been charged in the case, prosecutors said.

Yesterday's testimony was the first in the nearly two weeks since the government completed its presentation, which lasted nearly nine weeks. U.S. District Court Chief Judge Aubrey Robinson, who is presiding at the trial, permitted a week's recess to allow five defendants, who are acting as their own attorneys, to prepare their case.

In addition to DeBardelaben, 25, of Washington, and Vortis, 36, the nine men on trial include Warren Brown, 55, also known as Prince Asiel, the U.S. leader of the Black Hebrews.

The nine are charged with operating a "racketeering-influenced criminal organization" that prosecutors allege went on shopping sprees with worthless checks and stolen or fraudulent credit cards and trafficked in stolen airline tickets, primarily to support the sect in this country and in Israel.

Addressing the jury in his opening statement, James B. Stone, 37, one of the defendants acting as his own attorney, said "you will hear testimony that a common unity did exist between these men . . . but the common ideology is a belief in God and the unity is unity of oppressed people."

Nevertheless, Stone said, the nine are "very individual men . . . who were acting out their own individual destinies."

Stone said he would explain the real meaning of the term, The Nation, as the Original African Hebrew Israelite Nation of Jerusalem is also called. "The Nation is black America," Stone said, adding that its members "may be sometimes in conflict with the laws of man but are always in accordance with the laws of God."

Keels, called to the stand by DeBardelaben's attorney, Joseph Bernard, replied to Bernard's request that he state his name by saying, "First I want to give praise to God."

Robinson cut Keels short and told him to answer the question.

Attorneys in the case have said they will not base their clients' defenses on religious persecution or other First Amendment issues and Robinson has been a stern enforcer of that dictum. Keels also testified that he flew to Israel in 1983 on a ticket provided by DeBardelaben, but that he was refused entry by Israeli officials.