I use the word ideology meaning a system of beliefs that is immune to rational criticism and control. It makes sense to differentiate between three different kinds of such ideologies. I call them political, economical and religious. A typical political ideology is nationalism in all its colours and tints. Political ideologies had their heyday in the twentieth century when the belief in remaking the societies, political maps and human beings was very influential and powerful ideologies of communism, fascism and some of their derivatives were born and expanded their influence all around the world. The victory of the allied forces crushed both German Nazis and Italian Fascists, but strengthened for several decades the influence of Communism which divided into several distinct and competing branches. Then the Soviets overexerted themselves and their empire collapsed together with their ideology that has survived only in some marginal countries, sometimes just as a guise for a local nationalist ideology -- a typical example is North Korea. But in the wider world, political ideologies have been largely replaced by a mixture of classical nationalism and economism, an ideology based on the popular form of neo-liberalist economics. As in the recent past, it was believed that all the most important problems can be solved by means of political decisions and actions, it was, and still is widely believed that liberalizing the economy and monetarizing every field of human activity is the solution to all or most big problems humanity faces. The Friedmanian ideology has some similarities with the Leninist one, and it is exemplified in a funny way by some East-European ideologists who have once successfully professed the all-powerful Marxism-Leninism and now are using quite similar expressions about the virtues of free-market Capitalism.
The strength of an ideology can be measured by readiness of people to die or to kill in its name. Here, the modern free-marketist Capitalism cannot boast with such exploits as the Nazis or Bolsheviks. Friedmanians have no Auschwitz, no Katyn, no Cambodian killing fields. But when we delve deeper into history, we find enough examples of genocide, be it in Africa or in the Americas. Here, another ideology, religion has had its part. The colonizers and slave traders believed in their right to make profit, and they believed this right was given to them by God himself. The belief that was shared by Muslims whose behaviour in Africa and elsewhere was often as genocidal as those of their Christian brethren and rivals.
Religion is by far the most sophisticated ideology, and an ideology per se. It has also had enough time to evolve and to find ways to safeguard itself against every kind of dangers. Its forceful comeback in the last decades of the XXth century proves it has enough strength to influence the events in the world.
If an ideology gains power, it has the tendency to subordinate everything else. An ideology influences culture, human relations, everyday behaviour. Its language pervades everything. In the USSR we had "Marxist history", "Marxist science". In medieval Europe science and philosophy had to be servants of theology, of religion. Christians want Christian philosophy, Christian literature and art, Christian marriage and education. Islamic extremists want to insert into the textbooks of chemistry the phrase "If Allah wants". E.g. "Hydrogen and oxygen react, and the result is water, if Allah wants". I think one of the greatest achievements of human spirit and culture is freedom from ideologies. Unfortunately, very few people are ready to die (or to kill) for such freedom. Free thinkers can be very brave, but their following is always very limited. We are ready to reject an ideology, but most often just replacing it with another one.