Messiah, strictly speaking, relates only to the original series, where a serial killer sets out to kill twelve (actually eleven) men with the same names as the Apostles, in the same way that the original Apostles were killed according to tradition, e.g. St Bartholomew being flayed alive and St Peter crucified head down. The killer does this because he believes himself to be immortal, a second Messiah.

Messiah could probably claim to be the most gruesome detective thriller series on television, but also one of the more intelligent. I didn't see this first series when it was shown in 2001 (as a 12 year old I was far too young to watch this sort of thing), but I did watch the three further series which it spawned. However, since then, I've gone back and watched this to see how it all started off. It was originally based on a novel of the same name by Boris Starling, and he helped to write the screenplay, which probably means they're pretty similar.
Messiah revolves around DCI Red Metcalfe (played by Ken Stott), who is brilliant at getting inside the mind of a serial killer, but has his own personal demons to contend with, least of all his brother, recently released from prison and a bit loopy. He and his team (including Frances Grey, Neil Dudgeon and Jamie Draven) are on the trail of a violent killer in London, who chops out his victims' tongues and replaces them with a silver teaspoon. At first they cannot find any connection between the targets. However, they soon discover that the killer sees himself as the Messiah, and is murdering people based on the 12 apostles. The race is on to work out who his next victims will be, protect them from a grisly death, and piece together the clues to track the killer.
I would like to start by saying that those with a weak stomach should probably not watch Messiah. It is the television equivalent of David Fincher's Se7en, both in terms of intellectual, biblical patterns of murders, and downright nastiness. Each of the victims is killed in the manner of the 12 apostles, which includes one man being skinned alive, one crucified upside down, one beheaded, and one cut in half. Plus, with 12 apostles to go through, you can tell Messiah will have a pretty hefty body count. These are all brought to life with some very gruesome special effects. The story is split into two hour-and-a-quarter long episodes, called The First Killings then The Reckoning, and cramming all these crime scenes into such a relatively short space of time means that the pace never drops.
The acting is top class by everyone, but Ken Stott really stands out in the main role. Red is haunted by his past, in which he betrayed his brother and had a car accident, and he is constantly either shouting with rage or thinking intensely. His wife, Susan (played by Michelle Forbes) is deaf, and the silent scenes between them when they talk in sign language are brilliantly acted and really passionate, and something that I've never seen before on TV. Neil Dudgeon is also great as DCI Warren, who as well as helping with the case has to deal with his ex-wife, young son and gambling problems.
The mood of Messiah is very dark and moody as you'd expect from the subject matter and the complexity of Red's character. In some places it's quite creepy, a bit like Se7en, and borders on horror. The build-up to the discovery of a body is almost unbearably tense, and when the team are chasing round against the clock to get to the next victim in time it's quite exciting. I thought that the characters' side stories were handled very well; they really complimented the main story, and never got in the way or felt unrealistic. The flashbacks of Red's past were also done well; showing a bit more at a time to keep the viewer guessing.
Another trademark of Messiah - and most murder mysteries really - is the twist when the killer is revealed. They lead you one way, then there's a twist, then right at the end there's another twist that turns everything on its head, and I thought it was good how Red's background turns out to be more significant than at first it seemed (if you've seen it you'll know what I mean). I thought the ending was a bit rushed, but it was certainly exciting.
Messiah Series 1 was definitely the best of the four series. It's got the best plot (even if it is completely based on Se7en) and - I hate to be gruesome here - it has the best deaths. Also, the twist is very good, and at the time it was quite different, although nowadays there are more gritty police dramas than you can shake a warrant at. Definitely one to watch if you're a fan of Se7en, or a fan of crime dramas generally, as this was one of the best. Strong stomachs only, though.
Of the three series since, I think the last one, based around Dante's Inferno, was the next best, since it returned to the formula of classical references and killings in the style of a text, which was missing in series 2 and 3. So, if you watch this and like it, I'd recommend you watch series 4.
I don't know whether this counts as advertising or anything, but Messiah Series 1 (and possibly some of the others) is currently being shown on Telewest's Teleport service, so you can watch it whenever you want for free!