THE writer-director of Calvary (Cert. 15), John Michael McDonagh, opts for a second semi-colon in the film's opening quotation attributed to St Augustine: "Do not despair; one of the thieves was saved. Do not presume; one of the thieves was damned." Some sources omit it, making a world of difference to the meaning, not least in a film self-consciously released on the cusp of Holy Week.

But who is going to be damned in a film where a forthcoming murder is announced in the confessional box? Brendan Gleeson is the priest startled that he is the person to be murdered. This is Part Two of McDonagh's "Glorified Suicide Trilogy", as he calls it. Part One was his début film The Guard (2011), in which Gleeson plays another man in uniform, a Galway police sergeant. As with Calvary, we come to know somebody who is not always popular, and frequently unorthodox, but who speaks the truth in love.

It is precisely because Father James Lavelle is a good man that he is to be killed. His murderer was repeatedly and brutally abused by another priest, long dead, from the age of seven. The now adult victim, in seeking restitution, has deliberately selected a good priest to atone for a wicked one, and even for a Church with many sins to its name. He tells Lavelle to settle his affairs before execution on the beach the following Sunday at an unspecified time.

Most of the film thereafter concerns itself with James going about his village duties not far from Sligo. The screen is filled with a plethora of needy people, including a quirky butcher, promiscuous wife, atheist doctor, pastorally inept colleague, despairing barman, detective inspector whose predilection is rent-boys giving James Cagney impersonations, and a despairing tycoon who somehow now owns (rather like Dr No) a painting belonging to our National Gallery.

Despite, or perhaps because of, Lavelle's impending fate, there is something resembling divine comedy about all this. Not every laugh - and there are many - is tinged with gallows humour, but sight of the Atlantic shoreline constantly reminds us that this is to be the place of Lavelle's own Calvary. While there are Christ-like elements in Gleeson's agonising performance, if we are meant to see him as one of the thieves, then what exactly is it that he has stolen?

Lavelle is a widower whose suicidal daughter, Fiona (Kelly Reilly), visits him. She says that he robbed her of parental attention throughout childhood and even here in her hour of need. His perceptive straight talking divests parishioners of their self-delusions, which isn't always easy for them to forgive. And, as many a saintly figure is well aware, he, and the Church, has arrogated powers and privileges that belong to God alone.

Calvary owes something to other great films about priests, notablyHitchcock's I Confess, Bresson's Diary of a Country Priest (Georges Bernanos is cited at one point), and Melville's Léon Morin, Prêtre. Yet it remains masterly in its own right. Throughout the film we are left guessing whether Lavelle will flee from or face his killer - but also, more importantly, whether his death would constitute utter waste or holy sacrifice.

As for his would-be assassin, the film is a meditation on that second semi-colon: is he (are we?) redeemed or damned? Estragon (Samuel Beckett's Waiting for Godot) may think that for one out of two to be saved is a reasonable percentage, but is McDonagh more of a universalist? A clue may lie in his indebtedness to the Holocaust survivor Jean Améry's philosophy concerning the difficulties of forgiveness. If you go and see it (and I hope you will), be sure to stay for the credits, which offer some kind of answer to these questions.