President Reagan's ringing castigation of the Soviet Union as the ''focus of evil in the modern world'' doubtless pleased his evangelical audience in Orlando , Florida. But it may be asked whether this kind of dogmatic assertion - however sincerely believed - contributes either to dealing effectively with the West's Soviet adversary or to destroying the fallacious ideology of communism.
Mr. Reagan cannot be faulted for urging Americans to be wise to the tactics, manipulations, and persuasions of the last remaining empire on earth.Nor should watchful advocates of freedom throughout the world fail to be alert to the nature of materialistic communism. There is ample proof - in the experiences of Hungary, Czechoslovakia, Afghanistan, Poland, and many other places - of the ''aggressive impulses'' of the USSR and the continuing need for the West to stand vigilant and well armed in the face of formidable Soviet power. Realistically, it would be tragically foolish for the West to let down its guard , for Moscow will pursue its expansionist aims wherever and whenever it has opportunity to do so.
''Devil theories,'' however, have their pitfalls. They tend to oversimplify issues and therefore to distort reality. They incur the danger of giving power to evil. To exaggerate the ''evilness'' of the Soviet system is to weaken the sound basis of realistic assessments on which policies and strategies should be devised and carried out. It is to eschew a sense of balance, and to reject encouraging and building on the positive even in so ideologically benighted a land as Soviet Russia.
Can indeed a nation, any more than an individual, be all bad? There is a kind of moral self-righteousness that would claim so - that would ignore, say, the Soviet Union's desire for peace and its achievements in technological and other fields. Even its self-restraint abroad in some instances. The need, perhaps, is to make a distinction between the ideology of communism and the practice and the practitioners of it. Kremlinologists who have closely studied Russian and Soviet history and the Slavic character tend to believe that it is as much nationalism and historical experience that drive Soviet policy as Marxist ideology. Indeed, it is doubted that there are many ''true believers'' left in the Soviet Union. Marxism is falling of its own dead weight.
Which is not to say that the danger of the concept recedes in other parts of the world or that Soviet state totalitarianism is less of a danger itself. It is not. But it comes down to how one deals with the threat. An ideological crusade appeals to Mr. Reagan's fundamentalist listeners, who tend to see the confrontation with the Soviet Union in the stark terms of a black-and-white struggle between good and evil. Yet surely the aim of Western policies ought to be to foster those conditions of international stability and cooperation in which Soviet behavior can best be altered and in which the Soviet system itself can gradually evolve. Attacking the Kremlin as the main repository of the world's evil hardly seems calculated to elicit more cooperative attitudes.
One final point. Military might cannot destroy an idea. Marxism (and even that term has many definitions) will flourish only as long as it is believed to offer something worthwhile. When it is seen for the false, enslaving idea it is, it will destroy itself. The West's answer to Marxism lies not in facile rhetoric but in example - in the model it sets of moral and spiritual integrity. Let the Western nations cleanse themselves of their own lingering ''evil,'' their own materialism and im-moralities, their own greed, their own political, economic, and social deficiencies. Then will they persuade those who mistakenly cloak themselves in the vesture of communism - including the Russians - that there is a better way.