I wouldn't really say that atheism is a bigger movement than skepticism. Sam Harris, who is one of the most influential figures of the atheist movement, has already said at a convention of atheists that it is wrong to emphasize atheism rather than skepticism. Another influential atheist and skeptic, Michael Shermer, has always emphasized skepticism over atheism. And really, all atheists are skeptics, that is how one becomes an atheist, through skepticism about religion.

But as you say, it is because of the aggressive nature of theism in our world, that skepticism about religion becomes more important than skepticism about many other things that we can reasonably be skeptical about, such as UFO abductions, the Illuminati conspiracy, the dangers of putting fluoride compounds in drinking water, magic healing crystals, ghosts, reincarnation, psychic power, Tarot cards, palmistry, out of body experiences, etc. There is a more or less unlimited amount of nonsense for us to be skeptical about. But it is the religious people who cause the most social harm.

There is a whole renaissance in atheism, sometimes known as New Atheism although this is a misleading term, since it rests on the same foundations as the old atheism. However, following the 9/11 terrorist attack, a lot of people came to the conclusion that the time has come for the human race to rid itself of religion.

Lots of people have known for centuries, even millennia, that religion does not make sense. Voltaire knew, Oscar Wilde knew. In the 19th century, Robert Ingersoll wrote a devastating series of books which methodically destroy religion. He already said most of what needed to be said. The finishing touches were applied in the mid 20th century by Bertrand Russell. Between Ingersoll and Russell, religion was philosophically obsolete. And yet, the general public did not care. Philosophers cared, and the overwhelming opinion of serious philosophers at least from the 20th & 21st centuries is atheism. There are still theologians who like to pretend that religion remains intellectually respectable, but it doesn't.

So what the New Atheists have done is to drive home the point. They have explained atheism to the general public and made the kind of impact that Bertrand Russell did not. This impact is partially the result of the great eloquence and scholarship and passion with which the Four Horsemen of the Apocalypse, Christopher Hitchens, Richard Dawkins, Sam Harris and Daniel Dennett presented their case in the form of books and lectures and debates and youtube videos; it is also the result of increasing public concern about religion that resulted from the 9/11 terrorist attack, which was carried out by religious fanatics.

Many people, not enough people but many, have come to realize that the time for religious fanaticism has passed. It may have had a place in the middle ages, but we cannot really tolerate it in the modern world. It is time for the human race to grow up and to stop basing our lives on fairy tales. We can no longer be too polite to criticise people's religions, not when those religions are the motive for which terrorists hijack airplanes and crash them into office buildings, among many atrocities.

It is also a bit of an oversimplification to blame terrorism entirely on religion. European colonialism, which itself was always given a religious justification in terms of spreading Christianity to the heathens, has caused so much cultural disruption, economic disparity, environmental harm, and political abuse, that large parts of the world have developed a hatred for European based civilization, despite the fact that the age of European colonialism has really ended decades ago. No European nation has any substantial colonial holdings any more; England still has the Falkland Islands. The US, originally a European colony and now a dominant power on its own, has also acted in a heavy handed manner throughout the world, and is resented for doing so, even apart from religious differences of opinion, although that too has changed fundamentally with the end of the Cold War. But vast reserves of historical resentment exist in the Muslim world, going back as far as the First Crusade, a thousand years ago. That is still resented.

So in a sense, the 9/11 terrorism was not just an effort to assert the supremacy of Islam over Christianity or Judaism (although that was undoubtedly part of it). It was an effort to put the arrogant US in its place, to show that the US was not invulnerable to attack, and that downtrodden and oppressed people of the Earth can strike back against colonial powers.

But religion remains the essential motivating force behind international terrorism. Religion is the ultimate motive, which invokes the power of the afterlife. It would be much harder to recruit suicide bombers, or kamikaze pilots, if they did not believe the lie that they would be instantly transported to heaven, by dying on Earth as holy martyrs to the cause of Islam.

Even people who don't actually believe in Islam find it useful to pretend that they do, in order to strike terror in their chosen enemies, because the ruthlessness and fanaticism of Muslims has become legendary. Islam is the world's most dangerous weapon, more dangerous than nuclear bombs - and the time may still come when Muslim terrorists will use nuclear bombs. That represents the next level of Islamic terrorism. And the Muslim nation of Pakistan does have nuclear weapons. There is nothing implausible about the idea that they will sooner or later provide Muslim terrorists with those weapons. Pakistan, which was once part of India at a time when India was part of the British Empire, has great resentments to nurse. They have reason to hate the Western world.

Islam is not the only religion causing trouble in our world. Christianity is doing ridiculous things in America as well, currently having created a mass panic about transgender people using bathrooms. It is unbelievable. Several other religions are also causing trouble in their own way. Religious insanity is rampant in our world. Meanwhile we cannot even afford to have to deal with these things at a time when we need to devote our attention to very grave environmental problems.

So that is why it is religion, more than any other form of implausible beliefs, which concerns skeptics in the 21st century.