So, would it be your suspicion that some very large majority of those in US prisons today, who claim to hold to one religion or another, do not, and are just faking it for the perks?
I don't how strong I want to say my suspicion is, but ... yeah, that's my suspicion.
Our gardener is an ex-con who was by his own admission, an addict before he was put into jail. My wife's family has known him since he was little kid. He told me that, as a former addict, he was required to participate in NA (Narcotics Anonymous) sessions during parole after getting out. He used to go to NA meetings offered through Victory Outreach, a non-denominational Christian out-reach service catering to recovering addicts and ex-cons. He walked in on a drug-deal going down between the Pastor and a couple other folks, and managed to use that to his advantage in getting positive reports to his parole officer.
Another former neighbor, now in jail again, got out of prison and managed to get into Los Angeles' Catholic priest, Father Boyle's, "Home Boys" Program which runs a Bakery, Second-hand Furniture, Appliance, and Clothing store, and offers some training in each. That guy was a slug before he went in, a weasel while in, and a genuine reprobate, ... still is for all I know, if he hasn't gotten himself killed. He used his "claim" to having gotten religion on anybody that he thought would buy it, including me and my wife. I bought it at first and discovered that he was more clever than I was, about $1,000.00 later.
That's two anecdotes. I generalize from those, ... indubitably more than I should. Shame on me.