EVEN though Wednesday was not Christmas day for the Russian Orthodox in Moscow, it was fitting that Mikhail Sergeyevich Gorbachev resigned his post as president of the old Soviet Union on Dec. 25.Not to stretch it too much, but for the West over the past five years Mr. Gorbachev has been a bit of a Father Christmas figure - a totally new (look, ma, he smiles!) type of Soviet leader who globe-trotted with a hefty bag of perestroika and glasnost and each year yielded more gifts of freedom and human rights to the Soviet people while easing tensions between East and West. His greatest gift has been the end of the cold war and the emancipation of the East-bloc nations. He is truly a historic figure of the 20th century, an enormously gifted, complex man who towers above his 1980s contemporaries on the world scene. Given the kind of leader the Politburo could have chosen to succeed Konstantin Chernenko in March of 1985, we've been fortunate to have Gorbachev at the Soviet helm. He has so dominated the cycles of world news, and so much change has come by his leadership, that it seems he's been on top for a decade, not six years. New thinking and openness, nuclear arms reduction, more truth in media, Sakharov's release, "Gorbymania all are part of his legacy. Not that we're starry-eyed about Gorbachev. As a prot of former KGB chief and Soviet leader Yuri Andropov, he inherited a rigid, inefficient Stalinist system that was failing its people more than most experts in the West realized. Deep generational changes in attitude - towards Western ways - were taking place in an increasingly well-educated Soviet middle class. The ruble was weak. Tanks weren't needed. Consumer goods were. In the global village of the 1980s certain realities about the disparity between West and East could no longer be denied. Gorbachev thought he could reform communism - import a few Western notions to make socialist life a bit better. He could mix McDonald's and Marxism. But a little truth was more than the system could handle. Reforms were outstripped by a revolution. Gorbachev didn't adapt. Given this change, Gorbachev may have become irrelevant to Soviets. Still, he did the important work of addressing Western fears. The West may not have been ready for the "new Russia" that is emerging. Not even Russians know what will happen. But to a world weaned on apocalypse, Gorbachev has made the transition seem all right.