POLITICAL CONVICTION is still counted by the world as a virtue - but warily, and with the most emphatic exceptions. Conviction is the sustenance of liberty and justice, but not everybody's ideas about liberty and justice are the same as Jefferson's. Political conviction has its dark and crazy side, where argument is abandoned for the gun and the torch. The past few days offer a wide and melancholy selection of examples. They arise, as they frequently do, in the Middle East, and they seem to be the acts of people who fear that they are about to be pushed aside by history.

In the Iranian port city of Abadan, on Saturday night, several people carefully and skillfully set fire to a crowded movie theater, burning to death several hundred members of the audience. It was not an impulsive or accidental act. The arsonists had calculated purpose, and no doubt it seemed urgent and compelling to them. Some of the Iranians in this country have made the accusation that it was the work of the Iranian government's secret police. According to that logic, the government was attempting to discredit its opponents by staging a spectacular crime that would be attributed to them.

Perhaps. But it's also possible - and, on present evidence, it's more likely - that the fire was what it seemed. Ramadan, the Moslems' month of prayer and fasting, has begun. Moslem militants have held angry demonstrations in a number of Iranian cities protesting the widespread violation of the religious rules and the government's refusal to enforce them. A number of restaurants and movie teachers elsewhere in Iran have been firebombed since the beginning of the holy month - a ferocious warning to the impious. The disaster in Abadan appears to be another indicator of the tension that develops in a country that is attempting to transform itself, within a couple of generations, from a highly traditional medieval society into a modern technocracy.

On the day after the Abadan fire, several Arab terrorists opened fire on an El Al Airlines crew in London. They killed an Israeli stewardess, as well as one of themselves. The affair had been organized by the Popular Front for the Liberation of Palestine, the most consistently bloody-minded of the Palestinian organizations. The PFLP represents political conviction in a pure and intense form, rendered especially irrational and dangerous by the circumstances that it is also hopeless. It is wedded to purposes that it can never achieve and that have been, in fact, abandoned in one degree or another by most other Palestinians. The immediate purpose of the attack in London was presumably to incite a counterattack by Israeli forces that might derail President Carter's negotiations with the Egyptians and the Israelis at Camp David in early September. Israeli planes, as always, attacked a Palestinian camp in Lebanon.

In recent months a strange hit-and-run war has been waged, in both Western Europe and the Middle East, among various Palestinian factions and their respective backers in the Syrian and Iraqi governments. The representative of the Palestine Liberation Organization was assassinated in London last winter. That led to the attack on the Iraqi Embassy in Paris, several weeks ago, in which an Iraqi guard and a French policeman were killed. Society can protect itself fairly well against the people who take to the gun for money. The people who bomb and burn out of conviction are infinitely harder to deal with.