It's not always easy to keep up with President Reagan.

One week, as in Orlando, Fla., before the evangelicals, he is a thundering moralist who castigates advocates of a nuclear freeze for failing to enlist in his holy crusade against "the evil empire" of the Soviet Union.

Another week, he comes on as the most indulgent boss, winking at infractions, excusing, explaining, like a fond father who refuses to believe that any of his kids would steal a car.

In the Reagan theology, nothing is forgiven those who resist his call to build more arms.

This is not, according to him, a mere difference of opinion to which citizens of a free democracy are accustomed: it is a matter of religion.

"There is sin and evil in the world, and we are enjoined by scripture and the Lord Jesus to oppose it with all our might."

Who could ask for anything more black and white?

It is only when the circuit rider clatters up the White House driveway and climbs down from his horse that the gray sets in.

It seems that if you work for this administration, or are about to, the chances of your doing, or having done, anything wrong are practically nonexistent.

Even if you have been fired--or allowed to resign after strenuous semi-public maneuvering by the White House staff--you are blameless.

The president went to exceptional lengths the other day to gloss over certain facts about appointees or nominees whom many see as entering or leaving government service in something less than a blaze of glory.

Most astonishingly, he rewrote the history of the Environmental Protection Agency's John W. Hernandez Jr., who a mere three weeks ago became the EPA's acting director, succeeding Anne M. Burford, and who resigned last week. The president said that Hernandez "was not fired. Mr. Hernandez has been wanting to resign."

Wanting to resign? Just last Monday, after the president nominated William D. Ruckelshaus as EPA administrator, Hernandez wrote a letter saying that the honor of serving this administration "could only be enriched by the opportunity to continue to serve you under Bill Ruckelshaus."

To the president, critics of the EPA are small-minded people spewing out "unfounded allegations" that will never be proven by any of the six congressional committees investigating the agency.

Among the several accusations made against Hernandez is one that he allowed Dow Chemical to edit an EPA report on dioxin contamination. Neither he nor, apparently, President Reagan saw any conflict of interest here.

Similarly, the people who oppose the nomination of Kenneth L. Adelman as director of the Arms Control and Disarmament Agency are, according to the president, people "smaller than the person they are attacking."

In defending Adelman, the president again had to rearrange a few facts. His nominee, Reagan said, should not "be hung out to dry for having received a letter from someone else."

The letter in question, of course, is the famous purge memo that Gen. Edward L. Rowny, our chief negotiator in Geneva, denies having written.

Adelman could not help having the letter handed over to him; it is what he said about it later that has raised eyebrows. Initially he denied having "read" the memo--later he denied having "studied" it.

His account was eroded subsequently by the discovery of a note he had attached to the memo and sent to a political operative, presumably for action.

"Eyes only--Ed Rowny's confidential views on people," is what he wrote, indicating at least some knowledge of what was inside.

Senators don't mind if people plot purges in the disarmament agency, they just don't like being deceived.

When Adelman was asked at a Senate hearing if he had discussed agency personnel changes, he replied: "I had no thoughts about the personnel situation at all. I mean that very sincerely."

That kind of thing may trouble senators on the Foreign Relations Committee, but it doesn't bother Reagan the moralist. It is only when the Soviet Union comes into play that he can see evil.

Right and wrong have to do with communism and national security. Although many people regard the contemplation of nuclear war and the continued buildup of nuclear weapons as the greatest evil in the world today, Reagan apparently does not.

Unpreparedness apparently is a sin with him. The premium on people who abet "peace through strength" is illustrated by Thomas C. Reed, the deputy national security adviser who faces investigation by a criminal grand jury for allegedly making a stock market killing on insider information.

Other presidents might wonder about the propriety of keeping in such a sensitive post a man under a cloud. But not Reagan. He cannot bear to part with Reed until after the MX basing decision is announced. It's first things first with him.