Actually, it is a loathing underpinned by fear - the combination producing the kind of venom most eloquently and savagely expressed by HL Mencken; but replicated in a watered-down form by the New Atheists, self-proclaimed Skeptics and the like.
This is not a matter of merely rejecting religion; it is the business of making anti-religion into a focus of life; of not merely not practising religion but wanting the whole thing stamped-out, once and for all; so that it will be gone forever,
The reason is quite simple: cynics live only on their pleasures, and religion wants to take them away - or some of them at least.
The cynic finds no meaning, purpose or permanence in life - mortal life is everything, and the cynic's own life is (of course) primary, and the present moment and near future are the most certain - and this situation is only valuable (only tolerable) insofar as it is pleasurable (or at least not an active state of suffering).
The worst thing that can happen to a cynic is to suffer - therefore cynics favour humane murder (euthanasia) for themselves and others; and quick, painless suicide on demand ('assisted' suicide) as the bottom line, safety net in life.
But for the cynic, living is a matter of getting-through life as best as may-be; and this is achieved with the assistance of pleasurable habits and pastimes - like alcohol and other drugs, good food, travel, sports; and sex, as and when the chance arises, with whomsoever is fancied, and involving whatever activities are most enjoyable.
Religion threatens this whole package of a maximally-pleasurable and minimally-suffering life - thus religion directly threatens the primary coping mechanism of the cynic, the whole cynical modus operandi.
This is why cynics are hardline in their intolerance of religion - they correctly perceive that religion attacks their basis of existence; and for what appear utterly nonsensical reason! - yet a species of nonsense that carries extreme conviction and that most people find highly motivating - so that religious people will reject pleasures and choose suffering, in the name of their religion.
For a real cynic, religion is something rationally to be hated.