There are a few issues that must be clarified before dealing with the question you have posed.
It is important to understand that 'religion' and 'theism' are not synonymous terms.
Originally the term, 'religio,' meant mere "belief," "community," "shared customs."
Following are other secular definitions: knowledge and practice of our duties toward our fellowman; in character: studious, tenacious, conscientious, scrupulous, exact, strict, rigid, fervent, faithful, zealous.
Unfortunately, in the minds of most believers, religion necessarily involves theism, i.e., belief in a god,
There are after all non-theistic religions.
Religion can be and has been a very positive enterprise when practiced for the benefit of ALL mankind.
When it is founded on and supported by theism, particularly in a theistic multiplicity of warring concepts, conceived in the minds of cave dwellers, promulgated and proselytized throughout the ages, it is the bane of humanity.
However, when the positive aspects of religion are cited, credit goes, improperly, to theism whose tenets cannot ever be verified.
It is important to consider the meaning of the term, 'theism' as I understand it.
The dictionary defines it as "belief in a god or gods," personal or not.
Another: Speculative theism is the belief in the existence of a god IN ONE FORM OR ANOTHER.
This apparently alludes to the history of polytheism (believing in a plurality of gods) wherein gods took the form of anything from one-eyed frogs to heavenly bodies, to animals, to super-human beings, to gods of the wind, to gods of the seas, to gods of the earth, even extraterrestrials, on and on endlessly until, eventually, the thinkers and observers discovered so much nonsense and contradictions in the descriptive language defining those gods that each culture (not all, even today) finally melded them into one personal god.
However, the gods of each culture are different one from another--either slightly or extensively.
So, according to theistic language, we are still plagued by different kinds of gods.
Another: One may be a theist and not be a Christian, but he cannot be a Christian and not be a theist.
Another: Belief in a god with personal relations with human beings.
Another: Belief in a god with no personal relations with human beings.
Another: Belief in the existence of a god as the Creator and Ruler of the universe.
Much more can be said about the term, 'theism,' but the above should suffice for our purposes.
For me, the term, 'theism,' is synonymous with 'the LANGUAGE of theism.'
All belief is a form of meaning, and meaning is the basis of all language.
Were it not for the regulatory use of theistic language (the Bible, the Koran, etc.) by organized institutions, there would be no monotheistic (one god) beliefs.
In the history of theology, theologians (not alone philosophers and scientists) have consistently demolished the so-called proofs, for the existence of God, of their fellow theologians.
They are now left with nothing more than blind faith and the fallacy of ignorance: "You can't prove there is no God."
And that is true, if you define Him as unknowable.
But we can and have shown that all so-called proofs for the existence of a god, defined in non-verifiable language are epistemic nonsense.
Theistic language, among other indoctrinating mechanisms, has conditioned uneducated and unenlightened people, though even if well schooled, starting at the cradle, to accept its claims in the absence of evidence and in spite of the impossibility for verification.
Let us get directly to your question, then, to be read as, "Theistic LANGUAGE is the greatest crime ever foisted upon mankind."
First, let us discuss the "good" that the use of theistic language is said to generate.
I am not going to cite a host of specific acts of good which are attributed to the use of theistic language, in the guise of religious concepts, borrowed from the history of man's mythologies over the millennia, such as a sense of comfort derived from prayer, the happiness most people feel from believing in a god, following the Ten Commandments, the good works performed by clergy and nuns, feeding the poor, helping the injured, etc.
I shall argue that all such examples of good do not require that one adhere to the use of theistic language.
People all over the world who have no need for theistic language perform and/or benefit from those very same acts of good.
Such acts do not require belief in theistic concepts because they derive from a sense of human decency and concern for one's fellowmen, a sense that took countless millennia to evolve into a code of ethics.
Such acts of good were practiced many millennia before theistic doctrines and tomes were conceived, written, printed, and promulgated as regulating and controlling devices.
Theism has taken a proprietary claim to ethics, morality, and the language of both implying that without theistic language there would be no ethics or morality; i.e., God is the source of all good, forgetting of course, according to the Bible, that He is also the source of all evil.
Nothing in this universe can happen except by His Permitting Will.
God's Good encompasses acts that if man were to commit them, they would be called crimes of the highest order by our courts.
One need only read carefully and assiduously the language of the "religious" tomes of the ages.
Such THEISTIC LANGUAGE:
engulfs our society (not alone the world) which caters to believers to such an extent that especially in America, where with the constitutional right to free speech, non-believers are intimidated into silence on the subject and the big-business-dominated media and our schools refuse, are afraid, or, for financial reasons, choose not to give EQUAL TIME to an analysis of such theistic language,
prevents a proper education (as opposed to schooling, training, and rote learning) by denying freedom of inquiry, analysis, and reasoning in the schools about the meanings of religious and theistic terminology which permeates our literature,
conflates training, schooling, indoctrination, and conditioning with EDUCATION,
infiltrates our "secular" educational, political and judicial institutions with theistically inclined teachers and authority figures in seats of power.
deprives believers of freedom of thought by conditioning their minds early in life,
entraps the young defenseless human mind,
causes the development of closed mindedness,
closes the mind to facts and to reasoning about facts,
substitutes prayer for rational thought and action,
denies believers the freedom of will to read or study anti-theistic literature,
instigates and conditions early in life an unhealthy and naive mindset willing to accept unverifiable claims,
brainwashes the mind to accept nonsense beliefs without question,
retards development of rational thought by encouraging non-analytic and non-critical thinking,
forces people, through thought control, to believe what cannot be verified or falsified,
caused the death and/or imprisonment of innocent people declaring them to be witches and/or in league with the devil in order to enforce thought control,
causes people to accept superstitions,
confuses metaphor with fact and reality,
causes people to rely on blind faith instead of on their own abilities and potential,
causes people to fail to fulfill their full potential relying on a god instead of themselves,
follows the teachings of the Bible that keep women and gays "in their place" and preventing them from fulfilling their potential,
destroys self reliance (God will protect us.),
deprives people of freedom of action,
diminishes oneself by believing more in a god than in oneself,
causes people to rely on religion instead of medicine,
forces people to conform to preconceptions,
causes intolerance for differences in religious beliefs,
causes divisiveness among believers of different faiths,
causes racial prejudices,
causes people to accept myths as facts,
induces belief in gross falsehoods,
imposes its considerable persuasive power over politicians, political action, and naive minds,
forces government policies to be determined by religious concerns,
causes unhappiness in many obvious and subtle ways,
forces unhappy people to remain married,
causes people to support religious and indoctrinating institutions,
forces people to support the livelihood of the clergy,
persuades the poor to donate money, for the propagation and proselytizing of blind faith, they so direly need to support themselves,
has caused the expenditure of multi-trillions of dollars, throughout history, on "religious" structures, i.e., houses of the Lord,
caused and causes taxpayers to carry the load of taxes religious institutions should be paying for the services and infrastructure from which they benefit,
causes fear especially of death (in Hell),
causes feelings of guilt often by defining normal human behavior as "sin,"
uses fear, a sense of guilt, promise of reward, threat of punishment (among other conditioning techniques) to condition the unenlightened and uninformed, starting at the cradle, to accept its claims in spite of the impossibility for verification,
historically, caused the sacrifice of human beings to gods,
restricts ethical development,
has instigated, permitted, and performed some of the greatest crimes, cruelty, and  atrocities in the history of man in the name of God,
has a history of inquisitions--burnings at the stake,
causes religious prejudice and bigotry which so often result in uncivilized behavior, periodic ethnic cleansing, and religious wars,
gives intentional and unintentional support to white supremacists and terrorists,
causes an unacceptable increase in the population of the earth,
and, is responsible for an impending over-population of the earth, causing pollution, bumper to bumper traffic, deforestation, residential sprawl, over-crowded schools, shortage of food, medical supplies, etc., in third world countries, i.e., increasing harm to our environment, etc.,
casts suspicion on and lack of confidence in scientific development and pronouncements,
imposes its definition of 'life' and 'person' which are matters of science not theism, religion, or law,
has retarded scientific development and progress,
undermines and diminishes separation of church and state,
deprives clergy and nuns satisfaction of natural sexual drives,
advocates sex only for propagation,
denies use of condoms for safe sex,
deprives clergy and nuns of marriage,
causes the use of personal effort, time, and energy that could be more productively utilized,
causes people to spend their time, money, and effort preparing for an afterlife instead of for a fruitful and productive life while alive,
causes false hope, sometimes leading to suicide, in the hope of living, as a bodiless soul, eternally in "Heaven,"
sets the example (through biblical language) for committing evil acts, waging war, and mass murder (except for Noah and his family),
Denies non-believers from the possibility of high office in politics.
There is no question that, by accident, good often comes out of bad, even as bad often comes out of good, but throughout the history of man, no amount of good that theists have admonished us to perform, in the name of their multiple and various gods, has been able to undo the cruelty, atrocities, wars, deaths, mutilations, ethnic cleansing; social, cultural, racial, political, and religious divisiveness, thought control diminution of rational thought, financial hardship, etc., caused by theistic dogmas and language.