Derek Cianfrance's I Know This Much Is True opens in the same grisly manner as Wally Lamb's novel: with Dominick Birdsey telling the audience/reader of his twin brother Thomas, a paranoid schizophrenic, who one day, sitting in a public library and reciting a bible verse, takes out a knife and cuts off his own hand.
In the novel, Thomas performs this act as a sacrifice to appease God for the war in the Persian Gulf (by then, in October of 1990, the U.S. had interceded to halt Iraq's incursion into Kuwait). Thomas believed the U.S. had arrived to kill in the name of oil.
In the HBO series, Thomas simply recites a couple of bible verses before doing the maiming, beginning with the Lord's Prayer ("Our Father") and then reciting from the book of Matthew.
The chapter from Matthew is one of the most cited of the New Testament and contains a portion of the "Sermon on the Mount," a collection of ethical teachings delivered by Christ to his followers.
The citation is "Mathew 5:29-30," which, in the King James translation, reads:
"[29] And if thy right eye offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell. [30] And if thy right hand offend thee, cut it off, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell."
The verse has been pretty common in film and television. The Walking Dead's Herschel recites this verse as well in season 3, though, only for thematic purposes.
In I Know This Much Is True, the recitation involves a more literal act.
That the character who maims himself is named Thomas also carries a bit of biblical significance. Thomas the Apostle, doubting the resurrection, asked Christ to show him his hands, whereby he may prove that figure before him was the same whose hands had been nailed to the cross. In the series, it is Dominick who shows belief in Thomas and refuses to allow doctors to reattach his hand.
Look out for more indications of Thomas as a tortured, Christ-like figure throughout the series.