The first test posed to President Ferdinand Marcos when Benigno Aquino Jr., the opposition leader, was assassinated was to convince the people of the Philippines that he had had no hand in the killing. In the month since, Mr. Marcos has failed that test. He appointed an official and therefore--given his standing with the public--suspect commission of inquiry, which has done nothing to put down pervasive rumors of the Marcos family's complicity. His personal explanations have verged on the laughable: that his government did not know just what plane Sen. Aquino was returning on, for instance. A month's contemplation of the strange circumstances of the murder has left many Filipinos even more dubious than they were at the time.

The people, meanwhile, have left no doubt how they feel about Mr. Marcos' rule. Independent figures like Cardinal Jaime Sin and opposition political leaders have demanded an end to the Marcos regime and the return of representative government. Huge numbers of people have come out in the street, peacefully, to reinforce those demands. From one of those rallies the other day arose a burst of rioting that left 10 or more people dead. President Marcos was led to warn that he might suspend the modified emergency rule currently in effect and reimpose full martial law.

There have been suggestions that Mr. Marcos' conduct since the murder might make it embarrassing for President Reagan to make his long-scheduled visit to Manila early in November. Mr. Marcos gave new force to those suggestions this week. If Mr. Reagan puts off his trip, Mr. Marcos said, it could become difficult for the United States to keep using its two important military bases. It was an offensive remark. It was also a politically suicidal one for a dictator who has always portrayed himself as the single reliable guarantor of the Philippines' prized American connection.

Mr. Marcos' inability to protect Mr. Aquino had already raised a certain question about the wisdom of an early Reagan visit. The futility of the Aquino inquiry and the general deterioration in Manila would seem to settle the matter. Things do not appear safe. A Reagan trip, even an abbreviated one, could too easily be exploited as an endorsement of the way the unsteady Mr. Marcos runs the Philippines, to the detriment of the deep American interest in close ties with this traditionally friendly, strategically important country.