A Northwest Washington man pleaded guilty yesterday in D.C. Superior Court to armed robbery and assault charges stemming from four incidents in which church groups were robbed at gunpoint inside city churches.

Gregory Williams, 33, of 401 Edgewood St. NE, also is awaiting sentencing in Prince George's County after his conviction there of involvement in several similar incidents.

A second man, John Lucas, 32, of 3035 Stanton Rd. NE, pleaded guilty earlier this month to two counts of armed robbery in connection with two incidents in the District.

Assistant U.S. Attorney F. Joseph Warin told Judge George H. Revercomb that had the case gone to trial, the government would have proved that:

* On Nov. 20, 1981, Williams, armed with a handgun, entered the Mount Horch Baptist Church at 2914 Bladensburg Rd. NE and robbed nine people holding a Bible study class of $150.

* On Jan. 5, 1982, Williams and Lucas robbed about 20 members of a men's club at the Episcopal Church of Our Savior, 1616 Irving St. NE, of about $650 cash and credit cards.

* On Jan. 7, 1982, Williams armed with a handgun, robbed choir members of the Reid Temple AME Church at 1335 Michigan Ave. NE, of approximately $70.

* On Jan. 23, l982, Williams entered the Rhema Christian Center at 4915 Sargent Rd. NE during an evening service ostensibly to speak to Pastor Clarence Givens about performing a marriage ceremony, and then robbed Givens of $150 at gunpoint.

Williams pleaded guilty to three counts of armed robbery and one count of assault to commit robbery while armed, each of which carries a sentence of 15 years to life in prison. Revercomb set sentencing for June 8.