Chief Justice William H. Rehnquist, acting on an appeal by the Reagan administration, yesterday reinstated federal funding for religious organizations under a 1981 law that funds groups to counsel teen-agers to abstain from sex.

Rehnquist blocked a federal judge's ruling last April that said giving federal funds to religious groups under the Adolescent Family Life Act violated constitutional requirements of separation of church and state. Rehnquist's order remains in effect until the full Supreme Court rules.

U.S. District Court Judge Charles R. Richey, in his ruling here, said at least 10 religious organizations, including Catholic Charities in Arlington, had received federal funding for various pregnancy-prevention programs. Richey enjoined all funding of religious organizations, saying such funds have been used by religious groups to teach religious concepts.

Under the law, the Reagan administration has given about $30 million to various organizations nationwide, and approximately 20 percent of that went to religious groups.

Rehnquist, in staying Richey's order, said, "The issue seems to me fairly debatable, and I believe that there is a fair prospect that the court will ultimately reverse" Richey's ruling.

"It has been the unvarying practice of this court so long as I have been a member of it," Rehnquist said, to agree to hear "and decide on the merits all cases in which a single district judge declares an act of Congress unconstitutional. In virtually all of these, the court has also granted a stay if requested to do so by the government."

Rehnquist said that, "given the presumption of constitutionality granted to all acts of Congress," it is both likely that the court will hear the government's appeal here and "appropriate that the statute remain in effect pending such review."