Antitheism?
In contemporary activist circles bones are made over antitheism; that is opposition to belief in God. Historically Marxists and Anarchists took on an antitheistic stance because they saw theism as a mystical form of social deception which strengthens the existing order. However, with the modernizing energy of the late 19th and early 20th centuries deflated in the 21st, even revolutionary socialists slink away from making the critique of religion a central issue. This is especially the result of the failure to eradicate religion through naked repression in communist states and the resurgence of religion in the late 20th century when many observers thought it was going away.
Often activists in Marxist and Anarchist circles will consciously deemphasize antitheistic polemic and concern, proclaiming that religion is a private issue and that the antitheistic approach is a colonial one which must by definition disrespect indigenous spiritual beliefs, or further disrespect any and all religious people. It may very well be the case that a worthy cause will sometimes have to be put on the backburner. Anarchism requires the outright destruction of all states, but Anarchists might find themselves in situations where they can't publicly proclaim the destruction of the state and thus must instead focus on spreading Anarchism underground and/or involving themselves in day to day struggles. There are many parts of the world where declaring emphatically that religion is a sham will simply get you killed, so it would be unfair to expect Anarchists to in such situations to go engage in public antitheistic polemic.
Anarchists also can't afford to make participation in the movement conditional on irreligiosity since the masses of people are religious and Anarchism requires the participation of the masses of people. Even if Anarchism is to edge out religion by providing a replacement cosmology for people who are currently religious this substitution of eternal spiritual truths for historical political projects won't happen overnight. That said, Anarchism probably does need to retain antitheistic criticism of religion within its critical repertoire. We would do well to remember the reasons that Marxists and Anarchists historically viewed theism as a chimera to be exposed and overcome.
The point of revolutionary socialism is to focus on the corporeal needs of real human beings. Real human beings have corporeal needs appropriate to autonomous social animals; needs for material sustenance, needs for voluntary cooperation, needs for mutual recognition, needs for technological advancement, ect. These needs can all be catered to on their own terms by the application of the scientific understanding of nature via nature's transformation within the process of social production. When God is brought into the picture this ability of human beings to understand and transform nature and ourselves on our own and in its own terms is abrogated.
Human beings are now subordinated to a power above nature and above themselves. This power is usually supposed to have various moral rights to command obedience and certainly has the requisite force at its disposal to do. Humanity is thereby reduced from its status as an autonomous and rational social animal to a tool for satisfying the needs of a disembodied agency. Thereby theism requires a debasement of humanity that goes against modern ideas of human autonomy, embodiment, rationality, and value.
This is because religion was probably never meant to exist in the modern world. In prehistoric societies without a significant understanding of the workings of nature the idea of superhuman powers governing natural cycles and human social connections was the next best way of reproducing social norms and allowing individuals and societies to understand nature just enough to survive in it. With the development of significant social stratification where one social class came to rule another religion took on an additional function of justifying relationships of domination. The king rules and we must give him some of our crop because he is God, or has been enthroned by God. This all made sense in a world without modern science and based around the cycles of nature; principally the seasons.
Then modernity came along. The enlightenment and scientific revolution inaugurated a process whereby nature began to be understood in terms of impersonal and mechanistic processes immanent within nature itself (biological evolution, cosmic expansion, plate tectonics, ect.). People began resisting traditional authority intellectually and began to come together to wield mass agency in order to transform society according to precepts of justice, fraternity, autonomy, and equality. The result was that even though religion could still perform the cosmological function of ensuring social order, justifying inequality, and allowing human beings to interpret the natural world, it continued to be edged out in these functions by secular modes of thought such as science (social and natural), political philosophy and ideology, and moral philosophy.
The industrial revolution made it so society was no longer organized around nature's cycles, but instead became linearized according to homogenous and progressive social temporality (what Marxist thinker Walter Binyamin called "homogenous empty time"). God, the spiritual, and religion became just one among many cosmologies struggling for influence in a secular battle between human agencies for secular human objectives. These facts of modernity have brought out a contradiction between embodied human interests and theological modes of thought. In modernity we believe that we have to fight for a more ideal society on earth, such a society needs to cater to the material needs of human beings, and the autonomy of humanity is a core value. Meanwhile theology, inherited from a world based on natural cycles without any unified conception of mass political agency, or scientific understanding of the world, tells us that God's purpose is what's valuable, that we have to appease spiritual entities in order to survive and/or thrive, and that our fate is ultimately (often rightly) in the hands of superhuman agencies.
It seems therefore that consistent application of the humanistic principles of modernity (on which revolutionary socialism is based) requires that theism is ultimately a form of social deception which can only survive as long as human beings haven't fully assumed control of our own destiny. This is ultimately why Marx and Bakunin argued that in a more just society there would be no need for religion (what Marx meant by the idea that religion is the opium of the masses). Religion in modernity's most significant social function, since it no longer has a monopoly on justifying inequality, reproducing social order, or interpreting nature; is providing meaning and solace in a seemingly meaningless world. The modern world is characterized by a feeling of decadence such that the control of our lives by bureaucratic institutions, commercial interests, and repressive powers makes our natural and social existence seem inherently meaningless and hostile to our needs.
Religion is thus necessary for providing meaning to a meaningless existence ("the heart of a heartless world"). In a socialist society bureaucratic institutions, commercial relations, and repressive mechanisms are replaced with free associations among equal producers. Human relations take the form of mutual aid where they are structured for mutual benefit and free contract where relations are constituted by informally and freely structured terms. Thus overcoming class society in a free human community implies overcoming religion and thus theism.
Humanity comes to see itself as a corporeal social animal whose autonomy is not compromised by any superhuman powers. Meaning is restored to human relations since they become the intentional construction of the human individuals involved in them. Humanity would achieve meaning through itself and nature itself mediated by the progressive development of social production. Theism disappears.
Arguments against this view we saw above among Marxist and Anarchist circles are weak. There is nothing about thinking that humanity should move beyond religious thought patterns that should make one hostile to religious human beings. What makes one hostile to religious people is the willingness to suppress and persecute other people on the basis of religion, which Anarchists refuse by dint of our commitment to universal freedom of conscience. There is also nothing "colonial" about it. To say that antitheism is colonial is to say that indigenous peoples are essentially religious. In fact, consigning indigenous people to religious thought patterns by definition is to deny the indigenous their autonomy of thought and practice and to thereby continue the colonial project of utilizing essentialist views of indigeneity to set the indigenous completely apart from all other human beings.
Sources
Religion: A Very Short Introduction, Tweed
Economic and Philosophic Manuscripts of 1844, Marx
God and the State, Bakunin
The Philosophy of Atheism, Goldman
The Radical Humanism of Erich Fromm, Durkin
Gods, Benavides
Marxism, Day
Dialectic of Enlightenment, Horkheimer and Adorno
Modernism and Fascism, Griffin
Continental Philosophy of Religion, Burns
World Past to World Present, Stearns
Critique of Hegel's Philosophy of Right, Marx
On the Concept of History, Binyamin