President Reagan, leading a full-court press for congressional approval of the MX missile, told members of the House Armed Services Committee yesterday that "the only way we can reduce" military spending "is to reduce the need for it" through superpower arms reductions.

Defense Secretary Caspar W. Weinberger and Secretary of State George P. Shultz went to Capitol Hill at the same time as part of a concerted administration drive to win support of the embattled 10-warhead intercontinental ballistic missile.

"I must tell you, frankly, that cancellation of key programs, such as MX, will prolong negotiations, not facilitate them, and will reduce our ability to achieve arms reductions," Weinberger told the Senate Foreign Relations Committee.

Under a formula worked out last year, $1.5 billion for 21 additional MX missiles cannot be released until after March 1, after which two votes of approval by both the House and Senate are required before the money can be spent.

A White House official said Reagan has intentionally focused his appeals to Congress on the MX and his Strategic Defense Initiative missile defense system, known as Star Wars, rather than make a broad appeal for the defense budget.

Other officials have said that they expect Reagan to compromise with Senate Republicans on defense spending but that he wants approval of the MX in March to demonstrate U.S. "resolve" as the arms negotiations with the Soviets begin in Geneva.

White House spokesman Larry Speakes quoted Reagan as telling the lawmakers, "I would remind those who are concerned with the cost of national security, the only way we can reduce that cost is to reduce the need for it."

Earlier yesterday, Reagan spoke at the National Prayer Breakfast at the Washington Hilton. He recalled how House and Senate prayer groups formed in the 1940s, meeting "in the spirit of peace and in the spirit of Christ, but they need not be Christians."

He said members of the media know about these groups but have "generally kept it quiet."

"Some wonderful things have come out of this fellowship, a number of public figures have changed as human beings, changed in ways I'd like to talk about, but it might reveal too much about the membership."

Reagan said fellowships "have begun to spring up throughout the capital" and "exist now in all three branches of the government, and they have spread throughout the capitals of the world to parliaments and congresses far away."

"I wish I could say more about it, but it's working precisely because it is private," he said. "In some of the most troubled parts of the world, political figures who are old enemies are meeting with each other in a spirit of peace and brotherhood."

Speakes said he could not elaborate on Reagan's remark. "It will stay private," he said.