I heard someone on the radio refer to a particular group of people as being “a law into themselves.”
The only version of this idiom that I’m familiar with is “a law unto themselves.”
The expression derives from Romans 2:14. Numerous English translations of the Bible render the phrase as “a law unto themselves,” but some use a different preposition, like to or for. Here are three translations:
For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves.—King James Version (KJV)
Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law.—New International Version (NIV).
For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law.—English Standard Version (ESV)
The passage refers to Gentiles who, although they lacked the law that had been given to the Jews, had their own laws that taught them how to live righteously.
The preposition unto is a fossil. It once had several meanings, including some of the modern meanings of into, but, by the 18th century, its use had become obsolete in standard speech—except for the idiom “to be a law unto oneself.”
Note: The word unto, meaning to, is sometimes heard in an elevated religious context, as in “We cry unto the Lord.”
The modern use of “a law unto themselves” differs in meaning from the way it is used in the Bible passage. In the biblical context, the Gentiles are perceived as obeying a different law, but presumably their law applies to all of them.
In modern usage, “to be a law unto oneself” suggests that the person or people so described ignore laws that apply to everyone else, doing as they please, as if they were above the law. Here are some examples from the Web:
The NYPD may enforce the law, but they’re also a law unto themselves.—The Guardian.
They [the khap panchaya] believe they’re supreme, a law unto themselves.—The Times of India.
The Soviet Politburo was a law unto itself.—The Washington Post
The CIA: A Law Unto Itself—The Nation
Nicole Kidman – Law Unto Herself—New York Magazine
Here are some non-idiomatic uses that use the preposition into instead of unto:
French waiters are a law into themselves—TripAdvisor
“Hobby Lobby threatens to make religious believers a law into themselves.”—UCLA law professor.
“It’s a real policy dilemma because people begin to believe they can be the [sic] law into themselves.—Another law professor.
Note: The quotations from the law professors appeared in newspaper articles; the fault of substituting into for unto may lie with the reporters or their editors.
If a speaker does not wish to use archaic unto, the idiomatic choice of preposition is plain to:
French waiters are a law to themselves.
“Hobby Lobby threatens to make religious believers a law to themselves
“It’s a real policy dilemma because people begin to believe they can be a law to themselves