WE WRITE at a moment when it is not clear what President Ferdinand Marcos will do or what his fate will be. Certainly a momentous development is taking place; over the weekend the former defense minister and the acting chief of staff rebelled and called upon President Marcos to yield power to Corazon Aquino, on the ground that she won the election of Feb. 7. Yesterday's events indicated that the Philippines was teetering on the brink of something like civil war.

A couple of things, however, were clear enough. One is that Mrs. Aquino has, in a few brief months, developed unusual qualities of leadership, first by besting and then by conciliating her rivals within the opposition group, staring down both Salvador Laurel, who finally agreed to run as her vice president, and also the members of her own constituency, who didn't want her to run as an affiliate of his party. Next she further established her plausibility by: 1)holding this coalition together against all the betting, and 2)running a successful underdog campaign against an immensely tough and resourceful incumbent. Finally she took the hard decision to contest the fraud that has denied her victory and to organize a nonviolent popular campaign to that end. Her travail continues, but up to now this reluctant leader has grown and shown a gutsiness and a cool and an appreciation of democratic values that establish her as an authentic political personality.

Something else that is clear is that the military elements challenging Mr. Marcos can fairly be regarded as serious upholders of the democratic process and not as defiers of it. Gen. Ramos and Mr. Enrile are both men who, knowing Ferdinand Marcos well, finally broke with him. They broke not to put the military into politics but to take the military out of the politics into which President Marcos had thrust it, and to make the armed forces a fitter professional instrument in the pressing struggle against communist insurgents. Mr. Marcos accuses his former associates of launching a coup. He has it backward. It is his act of holding power despite his rejection by the people that constitutes a coup.

Finally, it is clear that President Reagan has been playing a startlingly bold hand. From an initial evident tendency to stick with Ferdinand Marcos, Mr. Reagan has moved to the point where a Marcos official yesterday suggested he was "licensing rebellion in a friendly country." This was a reference to his remarkable Saturday statement saying that official fraud had undermined the Manila regime's legitimacy and capacity to govern, and this was before his statement yesterday that aid would be suspended if Mr. Marcos used his American- supplied arms against his own people.

But President Reagan has not been "licensing rebellion." He has been affirming the democratic process in effort to assure that the Philippine people will be masters of their own destiny. This is the right policy.