Religion has been a powerful tool with different dimensions. We can start with Marxism 101 by saying that "Religion is the Opium of the Masses". This is the simplest form of oppression. Religion is a tool played by the elites to control the masses. It plays on fear and feeds on isolation. The "dozes" of faith blocks a person from looking at the world as it is and forces the individual to look at the "methaphysical realm" to avoid the problems of the earth, and worse, the power of the individual to self-emancipate as well as trigger a domino effect of a progressive wave.
History and Evolution of Ideas
Nietzsche, the legendary atheist philosopher, argued that faith in the icons of religion is "projections of fear into the unknown. He made it clear in his book: "Thus Spoke Zarathustra". A major climax in his various attacks on religion, specially in this book, is when the clown fell from the ropes and tells the prophet that the devil would take his soul by the ropes and dies. Fear of the unknown has been a major factor to say the truth. The easiest proof to look from that dimension is to look at the human civilizations 7000 years ago, and now. In the past, one of the oldest known societies, the Sumerians (used to be present in the current Iraq), worshipped plenty of Gods. The oldest script "Epic of Gilgamesh" reflected the fear factor of a civilization whereby anything that changes in an environment without any explained excuse, the phenomena was given a God or Goddess. Actually, the people at that time, they believed that Gods controlled every thing whenever those immortal mighty beings gathered up in an a Divine Assembly, and decided every single event in the course of humanity. Actually, they were so strong, that the Gods themselves hid on the tip of the highest mountain in heavens to hide from the great flood which they created because at a certain stage human noise gave them headaches.
As knowledge progressed, things became different. You would find Zeus, at the Greek mythology, telling Athena (the Goddess of Wisdom) that why humans blame the Gods for everything that happens to them (Homer: The Odyssey: Book I). Human will entered to influence the fate of an individual. This is to reflect that sciences evolved during the Greek times. Thales would invent a major mathematical theorem, while his ancestor Euclid's would invent Math. Pythagoras based everything on numbers (including the heavens) while Aristotle attempts to prove that the universe always existed and dared to apply the sciences of logic and philosophy on God (theory of the unmoved mover: where by God moves everything but never moves or else there is a stronger being that moves God). Democritus discovers the tiny particles that makes the whole universe, and discovers Atoms. Eventually, Epicurus invents the concept of "living for the moment" and "pleasure is the absence of pain" principles. With the Romans, the Gods remained powerful, but some thinkers dared to limit the role of the Gods as non-intervening in human affairs, such as Lucritius. Lucritius dared to merge Epicurus and Democritus to argue that there is no after-life since "spiritual atoms" would leave the bodies through the space volume of the atoms of the physical body, and the first wind would scatter those "spiritual atoms" into nothing. Best solution for the Roman philosopher, to enjoy life as much as possible since a person lives once in a life time.
Going to the Present
It can be noted that the effects of Gods declined as science progressed. You always had a Virgil to write an "Aniad" where the Gods would battle each other. Yet, it was a very well known fact that people like Virgil, despite their remarkable ability to write epics, wrote for political reasons. Virgil for example was a great friend of Emperor Augustus, the adopted son of Julius Caesar.
With the Revolution of the Christian Ideology, materialism was fought. The rise of Christianity threatened the Materialistic world, especially when a story of Jesus entering a Jewish temple and attacks the business merchants on it. Alan Woods called Christianity as Naive Marxism. He concluded that in his article "Marxism and Religion", that the first "Christians" of the Christian revolution failed to link their "anti-materialism" ideology with their means of productions, and hence the elites transformed the ideology into a much more advanced institution of dominion of the elites on the masses. Even though Christianity started different than probably the Codes of Hammurabi of "thou shall obey" whereby the ruler is the defender of principles but only he enjoys life, it ended as a means of oppression. For a Marxist, of course, directing attention to the metaphysical world rather the real world is wrong, but at least there was an attempt to break away from Materialism. Again Alan Woods' "Marxism and Religion" is a great source.
Impacts of Religion
As a role of using Religion to dominate the society, the ruler was always portrayed as the defender of the Sect. Whether we are talking Christianity' Judaism, Islam or any religion, the ruler was the divine defender of the Sect. People were always looking up to the skies in order to forget their problems on earth.
Second, Religion divided the Proletariat. Whether in the Past, where the masses were divided into hundreds of Gods, or now, to hundreds of thousands of Sects of the same Religion, the leaders depended on the isolated sect and promoted their defense since each individual is what he/she is based on their Sect on their identification card. Lebanon, the host of 18 official sects, got political leaders for most of those sects who are supposedly defenders of such a Sect. In the United States, a president can't be a Greek Orthodox since the system is not based on merit (actually racism here plays a nice role as well). The Zionists fought with the Socialists to push their sectarianism into racism by imposing the Jews as a race rather as a Sect. This led Karl Kautsky to wonder if the Jews are a Race and Marx attacking them in "On the Jewish Question."
Third, Religion became an imperialist tool as well as a path to racism. We had a Rasputin dominating the whole Tsarist Russia due to his healing abilities of the Tsar's son, something that pushed the Nicholas Second to assassinate him in the end. The Pope of Rome waged Imperialist wars through the Crusades , while Europe used the Middle East sect minorities as an excuse to invade it (like wise with the United States on a later stage). Several nations choose minorities in Lebanon, Syria, and Palestine to protect in order to maintain imperialistic interests (a process that started in the late 18th Century). Currently, the United States with the absence of an enemy ever since the Cold War ended, relied on Sectarianism to invent a new enemy, which was radical Islamism (something they created in the face of the Soviets, for the record). Bin Laden and his radicalism was the perfect excuse to trigger a chain reaction series to despise the Arabs on a larger scale (actually this perspective of racism to the Arabs always existed, but not to this extent... Edward Said discusses this in his book "Orientalism").
Fourth, Religion depended on Faith. A lot of scientific people are believers; the point is how much they are controlled by religion as an institution. I already mentioned how Religion is a tool to block self-emancipation, but the reliance on superstition is a strong argument. This would lead to the next section: the Solution.
Revolution Against Religion
Probably the most radical article to depict the formula mentioned In point four was Leon Trotsky's article, which he wrote for Pravda in 1914, "Vodka, the Church, and the Cinema" He clearly expresses the formula that the religious clerks extensively depended on superstition not only to impose faith, but to control the individual. The more the person is superstitious, the more he/she were controlled by the Church (or whatever equivalent institution). The logic to face that syndrome is not by force, like Stalin followed by closing down the religious institutions. What Stalin did in his "Soviet Empire" (rather Union) enforced religion among the individuals in a secret manner. Trotsky's formula stresses on the theme that the "faithful" or "superstitious" should be pushed away from Religion through education. The example back then in 1914 was the Cinema, in order to educate the Proletariat and impose the Cinema and its documentations away from Religion. This of course is not a tool by itself, but a means to a greater end. This end is of course the unification of the Proletariat despite race, gender, nationality, and RELIGION. This end can't be approached without the emancipation ideology of why it is needed to break away from superstition. The internationalist aspect of Marxism for example is a binding factor against the divisions of the different sects of Religion.
Just as the Cinema is a tool to assist the spread of secularism back in 1914, currently, for example, in locations of spread different religions, Civil Marriage is a wall-breaker to open the route of at least different sects can marry and making it possible without the interference of the religious institutions. Lebanon, for example, is in desperate need of such a mechanism, since it hosts 18 Sects. Love, again, is personal and not a religious belief. People who are basing their "traditional marriages" based on the same sect are indirectly committing Sectarianism. Religion is meant to remain at home, among oneself.
Some may argue that this is a Utopian process. It is not true, no goal as large as this can ever take place so easily. There is always a long route. Some may consider Atheism as an option. This also is not true. Atheism is also a form of belief. I am an atheist myself, but attacking people for simply believes is wrong and evolves to become reverse atheism (or simply a Sect on its own). Marx always argued, especially in his book "The German Ideology" that the base of change is through social interaction. The real solution to the problem of Sectarianism is a progressive revolutionary ideology which binds the Proletariat against their oppressors. The elites have different tools to dominate the Proletariat; the current religious institutions are part of them. In the end, people are people, and it is the culturals of the elites that divide the Proletariat. Lenin always argued that the ruling ideas of the societies are the ideas of its rulers. I agree, the idea is to smash those ideas and unify the masses. For me, there is no war but class war, other reasons which include religion wars are just curtains to hide the real wars, meaning class wars.