Religion, Christianity included, functions as a tool the powerful use to control the masses — promising pie-in-the-sky rewards in heaven to keep the poor docile while the ruling class benefits in the here and now. As Marx put it, it is the opium of the people.

Why people find it persuasive

There is real historical instance of religion legitimating existing power (divine right of kings, slaveholders citing scripture, established churches allied with monarchies), and it is a useful function whatever else is true of it.

Important distinctions

• • Religion as sometimes used instrumentally by power vs. religion as intrinsically or exclusively a tool of power.
• • A functional account of religion's social effects vs. an argument about its truth-value (functional utility to power says nothing about whether the underlying claims are true or false).
• • Cases where Christianity legitimated the status quo vs. cases where it subverted and overturned it.
• • Marx's specific historical target (mid-19th-century German state Lutheranism) vs. Christianity globally and historically.

Christian response

• • The genetic and functional critique commits a fallacy even if entirely accurate about sociology: showing that a belief is socially useful to some party says nothing about whether the belief is true — Marxism itself was famously useful to Soviet and Maoist state power in legitimating one-party rule, which by the same logic would make Marxism false, a conclusion Marxists reject, rightly, as a non sequitur.
• • The historical record includes at least as much religiously motivated subversion of power as legitimation of it: the Hebrew prophets (Amos, Isaiah, Micah) denounce the ruling classes for exploiting the poor in the name of God, not on behalf of them; Christianity was for its first three centuries a persecuted minority movement with no state power to serve; and liberation theology, the Black church tradition from slavery through the American civil rights movement (Martin Luther King Jr. explicitly grounding nonviolent resistance in Exodus and the prophets), and the Catholic Church's role in Solidarity's resistance to Polish communism (John Paul II, 1978-89) all mobilized Christian belief against, not for, entrenched power.
• • William Wilberforce's decades-long parliamentary campaign against the British slave trade (culminating in the Slave Trade Act 1807 and Slavery Abolition Act 1833) was explicitly and self-consciously religiously motivated, directed against a hugely profitable economic and political establishment, the opposite of docility-inducing opium.
• • If Christianity functioned straightforwardly as ruling-class control, one would predict its strongest growth among the powerful; instead sociological and historical study (Rodney Stark's work on early Christian growth) shows the movement grew disproportionately among the poor, slaves, and women in the Roman Empire — groups it empowered and dignified (giving slaves and women full standing at the Eucharistic table) rather than merely pacified.
• • The instrumental usefulness of a claim to some ruling interest, where it occurs, needs to be argued case by case against the specific historical evidence, not asserted wholesale as a structural fact about 'religion' as an undifferentiated category — Christianity's actual historical relationship to power has been far more contested and two-directional than the slogan allows.

Christian counterresponse

That is accurate and important, and most serious Christian social ethics since at least the 20th century (Catholic social teaching from Rerum Novarum, 1891, onward; the Protestant social gospel; liberation theology) explicitly names and repudiates exactly this co-option, treating it as a corruption of the gospel's actual content rather than a hidden truth about it.