President Reagan's top advisers have concluded that he has devoted enough emphasis to religion in his reelection campaign and must shift focus to major issues such as the economy, administration sources said yesterday.

"There's not that much more we need to do. We've laid out our religion line, and we're solid with that base," a senior White House official said.

Reagan advisers reached this conclusion as Democratic presidential nominee Walter F. Mondale prepares to deliver a major speech next week in which he is expected to criticize Reagan for mixing religion and politics.

Mondale advisers said yesterday that the issue and preparation of the speech are "very delicate" because they want to avoid offending key sectors of the electorate or leaving the impression that Mondale is insensitive to some evangelicals' views.

A Mondale adviser said the speech's goal is "not to raise this to a cutting issue" in the campaign but to "define" how Mondale feels about it and stress his support for a strict interpretation of separation of church and state.

Reagan, speaking yesterday to the Catholic Golden Age Association, a senior citizens group, did not mention religious themes that he has articulated in recent weeks. Instead, he discussed such government programs for the aging as Social Security and Medicare.

Religion is "not going to be an issue more than it has been," the president's campaign director, Edward J. Rollins, said yesterday. "I think Mondale runs a real risk if he tries to make it more of an issue. It's not a campaign theme of ours.

"We have that constituency, and we will reinforce it as necessary. But the less debate on religion the rest of the campaign, the better," he said.

The senior White House official said that some Jewish leaders have reacted negatively to Reagan's emphasis on religion in recent weeks but that the president would attempt to address their concerns in a speech here Thursday to B'nai B'rith. Mondale is also scheduled to speak to the group.

Reagan-Bush campaign spokesman John Buckley said Mondale would be making a "major blunder" by raising the issue. He said that Reagan has expressed his views about religion in "the Judeo-Christian tradition" and that "it's a very risky game that Mondale, [New York Gov. Mario M.] Cuomo and [vice-presidential candidate Geraldine A.] Ferraro are in when they seem to be saying who's a better Christian."

Buckley was referring to Ferraro's statement July 12 that she does not believe that Reagan is a "good Christian" because of his "terribly unfair" and "discriminatory" policies.

Cuomo recently criticized Reagan for having "wrapped himself in religiosity" and having "used religion aggressively as a weapon, as a tool." Cuomo, a Roman Catholic, has also differed openly with New York Archbishop John J. O'Connor over the role that the abortion issue should play in politics.

Reagan told a Dallas prayer breakfast recently, "Politics and morality are inseparable. And as morality's foundation is religion, religion and politics are necessar- ily related . . . . Without God, democracy will not and cannot endure."

At the same time, Sen. Paul Laxalt (R-Nev.) has sent a letter to several thousand ministers. Addressed to "Dear Christian Leader," it asks them to organize a voter-registration drive.

Reagan won the overwhelming support of conservative fundamentalists in 1980 when they first became a major factor in presidential politics. In the past, he has devoted the early stages of campaigning to addressing them and shifted in the final stages to major issues.

The officials said yesterday that Reagan probably will follow the same pattern this year, using the rest of the campaign to address the economy and other themes.

But, the sources said, Reagan strategists also realize that the president will hold fast to his beliefs on religion and such issues as abortion and school prayer and will talk about them when asked.