Bishop Smallwood E. Williams, describing Marion Barry, the onetime controversial black activist, as a "prodigal son" capable of redemption, called on other city ministers yesterday to join him in supporting Democratic nominee Barry's candidacy for mayor of the District of Columbia.

Religious allies of the Republican nominee, Arthur A. Fletcher, voiced a more skeptical view of Barry's transformation from a street activist a decade ago to a leading candidate for mayor with strong support from the city police officers union businessmen and some ministers.

"A rose is a rose is a rose," the Rev. A. L. Colston, pastor of Glendale Baptist Church told his congregation, in an obvious allusion to Barry. "And a scoundrel is a scoundrel, no matter where you get him from."

"The same police that are backing him (Barry) were locking him up weekly (before). The same Board of Trade that is giving him all the money, was asking the police to lock him up (then). The same ministers who are inviting him to their churches (now) were asking him not to come."

The tug-of-war over Barry's image became a central focus of campaigning yesterday morning as the two major candidates took their efforts to the midle-aged, church-going and politically moderate black Democrats believed to be a backbone of the city's electorate.

Yesterday's activity, marking the start of the final three weeks of the campaign, also set the stage for the climax of several weeks of behind-the-scenes maneuvering between the two cand dates in a battle for the endorsement of church leaders. Later today, Fletcher is expected to unveil a list of ministers supporting his bid for mayor; Barry has a press conference scheduled for a similiar pupose tomorrow.

Politicking with the blessings of the pulpit is a relatively new phenomenon for both men in this election, even through both won their respective party primaries last month.

Barry virtually conceded the church vote in the primary to his two opponents, Mayor Walter E. Washington and Sterling Tucker, and is just now beginning to get some of the ministers who supported his foes to line up behind him.

Fletcher, running for office in Washington for the first time, had only token opposition and exposure during the primary and now is beginning to campaign in earnest.

For both candidates, it was a morning when political messages were squeezed in between the rocking gospel selections of robed choirs who energetically clapped their hands and swqyed from side to side as they sang. Aprovals were voiced in shouts of "Amen." Barry signed a few autographs after church.

Williams, pastor of Bible Way Church, 1130 New Jersey Ave. NW, is considered one of the most influential ministers in the city because a great majority of his 5,000 church members in the city tend to folllow his political lead.

"After he's mayor, I'm gonna talk to him and if I see him gettin' outa line, I'm gonna say, 'Now watch out Barry,'" Williams said, later adding, "The prodigal son walked out of light into darkness . . . But one day, the Bible said, he came back."

Williams' sermon also continued words of warning to Barry about legalized gambling in the city, one issue that has drawn ministerial support away from Barry.

Barry, who is officially in favor of a referendum on legal gambling, played an integral part in setting up the city study commission that recommended that the District of Columbia launch a legal numbers game.

"We ought not try to make Washington, D. C." a Las egas we ought not try to make it an Atlantic City or Rno, Nevada," Williams said. "Let Washington be an example."

Fletcher, accompanied by his 31-year-old son Paul, appeared at two churches yesterday. Each time, he emphasized that he was a "family man" with six children who is strongly opposed to gambling, prostitution and marijuana use - things he said created "temptations" for young blacks.

Barry has vavored easing the penalties for marijuana use but has not made any public statements that he can recall favoring prostitution.

"You want a man in office who knows how to mold character, who understands he is a model for your children, his children and everybody else's children," Fletcher tol the congregation assembled beneath the high, walnut-paneled arches and aging grey steeple at Mount Carmel Baptist Church, 3rd and I streets NW.

"Two people are running for office, he said. "One of them has the image and lifestyle of being respectable. I know which one that is, I don't know about the other fella."

At the Glendale church, 4504 Gault Pl. NE, Fletcher read from a printed "creed" he developed for "The Society for Victorious living," a church-based job skills development project he planned to launch shortly after he resigned as director of the United Negro College Fund three years ago. The program never got going fully.

Copies of the creed, signed by Fletcher and printed on parchment over a picture of Jesus Christ bearing the cross, have been given to some ministers in town. Typical of the cred's tone is its beginning, which reads, "I believe my living will not be in vain because I am convinced that there is a God and I trust in his love, justice and mercy."

Barry also appeared at two other churches yesterday - New Southern Rock Baptist Church, 750 Buchanan St. NW, and Mount Bethel Baptist Church, 1st Street and Rhode Island Avenue NW.