Blind and total faith in any idea or person is dangerous and that's been proven time and again throughout history. In Hitler's book, Mein Kampf, he talks about the need of the people to eradicate the Jews, how they are the source of the woes of Germans, how blonde-haired, blue-eyed Aryan humans are the superior race and if these are allowed to breed out all other races there will become an Edenic, utopian German society with pure values at its core. And Hitler, of course, thought himself the man to lead this revolution. He instated death camps to exterminate the disabled, Jews, those of inferior races; he propagandized unfounded pseudoscientific theories on other races; he ordered experimentation on children; he instated a totalitarian state with a delusional cultural-religious ideal with him as its god-head, and any dissent or deviation was severely punished.
During the fascist movement in Europe, Mussolini, who was like Hitler also raised a Catholic and grew to despise the Catholic church, created a similar environment in Italy. And in fact, during the extermination of the Ethiopians (back then called Assybians) in north Africa, Mussolini gained the favour of the Catholic Church who supported him in his campaigns. In fact, the Vatican at the time declared it a justified "holy war".
Religion: the belief in and worship of a superhuman controlling power, especially a personal god, or gods.
In the German's cases, that power was Hitler, Nazism, and the Aryan creed. In the Russian's cases, that power was Stalin and Stalinism. The pattern continues. In fact, these leaders not only considered themselves as messengers of some radical, transformative ideals which would change the world but which also required absolute obedience and a totalitarian authority, they also persecuted anybody they perceived to stand in their way, whether those people were Jews, Christians, atheists, scientists, farmers, women, children or infants. These are men of absolutist beliefs, who hold extremely black and white moral views, practice extreme prejudice and total ruthlessness, and tolerate no dissent.
And had we, as secular nations full of free, multi-religious and multi-cultural peoples, not fought these oppressors for the freedoms to live our lives under our own personal individual liberties, opposing tyrannical regimes so that free people could live alongside one another with civility, autonomy, and equal rights for all, then these fanatical preachers, like Hitler, like Stalin, and their corrupt and deluded ideals, their totalitarian regimes, would have overcome us, and we would be worshiping not the God of the bible or the Allah of Islam or the Buddha or nothing at all, in freedom and alongside one another: no, we'd be worshiping Stalins and Lenins and Mao-Zedongs and Hitlers.
One such country, were men like these were allowed, unopposed by secular ideals, to become gods in the minds of the people, exists today: North Korea, where Kim Jong Un's grandfather, Kim-Il Sung, a man who has been dead for over twenty years, is still the official head of state, and is worshiped like a god.