When Matt is poisoned by Carbon Monoxide, he becomes a religious fanatic and never stops babbling about the word of God, although beforehand he wasn't the slightest bit religious. In his opinion, those who sin end up dead and those who live are true believers. He tells everybody not to mourn people who are dead as they were sinners. Everybody considers him mad, even Archie who was his closest friend during the disease. He is extremely obsessive with The Lamb and has a never-ending song being played in the cathedral. He also sacrifices kids to the monster Wormwood to appease him.
Matt appears early on in The Dead after being found unconscious in the Rowhurst School Chapel due to Carbon Monoxide poisoning after he and his gang of kids who were with him started a fire in the chapel. After he had been saved by Jack, Ed, Bam and the other kids in Rowhurst he told them that he had seen visions of The Lamb and that he was going to save them all.
After they have been brought to Saint Paul's, Sam and The Kid are given a massive feast but soon after that The Kid is locked up because they believe him to be "The Goat", which is a trickster and a demon. When Brendan turns up at Saint Paul's Cathedral which is where Matt is staying, looking for shelter after being exiled from The Tower of London, and being annoyed with Jordan, he tells Matt about Sam and The Kid and about Jordan. Matt sends Tish and a group of her friends to find Sam and The Kid believing them to be "The Lamb" and "The Goat."
When Matt tries to find out The Kid's name and he refuses to answer, he whips him. When The Kid finally answers and says his name is "Angus Day", Matt get's confused and locks both Sam and The Kid up in a cage, as Angus Day is the real name of The Lamb. Matt takes both of the children into a warehouse where they keep Wormwood. He asks them which of them should be sacrificed. When Sam answers that he himself should and The Kid says that Sam should, Matt throws The Kid in with Wormwood to be sacrificed. He does this because "The Lamb" would allow himself to die instead of his friend where as "The Goat" would not.
When Ed and his crew arrive to save Sam, Matt says they should go somewhere private and talk. He tells them that he didn't kidnap Sam nor did he kill The Kid.
Matt along with Archie, Ed and Sam are taken down into the Warehouse to see if The Kid is still alive by Ed's orders. When both Wormwood and The Kid aren't there Matt says that its a miracle as "The Goat" and the Fallen Star have both disappeared. When they walk back up to the Cathedral the rest of Matt's kids block the way and demand Matt back. However most of them don't look like they would fight Ed's team who are far better fighters than they are. When Ed asks if they is anybody whose willing to go with them to the Museum and nobody reply's Matt says that they are faithful to "The Lamb" and not just because of the fear of being sacrificed to Wormwood or the bargain of years worth of food from the "Tree of Life". However Charlotte, Brendan and Tish go with them. When all of them go back to the surface an armies worth of sickos are in the streets and have broken past Matt's barrier. Matt orders his kids to fight back to the Cathedral and forgets all about Sam.
Matt is first seen by Will, Ryan and Finn when they return to explain the situation of Saint George to Jordan and The Tower kids. Will comments on how thin he looks, saying that his ribs were all visible. After the attack on the cathedral Jordan decided to step in and help him, provided that he behaved himself and didn't try to sacrifice anyone. Matt himself realised that he had gone too far and toned down his preaching slightly.
When Matt and his acolytes reach the Houses of Parliament they decide to stay in a nearby church, saying that they felt more comfortable there. Shadowman begins to suspect that Matt is hiding something, and begins to follow him closely in the same way he did with Saint George's army. He watches Matt beating himself with a belt, muttering things that didn't make sense, and slowly realises that he has seen similar behaviour in Wormwood. He concludes that Matt has been bitten by a sicko and forces him to show him the bite.
Matt begs Shadowman not to tell the kids, which he agrees to. In the final battle, the sickos avoid Matt and are scared to come near him. This and the fact he remains (partially sane) after becoming infected suggests he is immune. Later on in the book, when the battle is over and Saint George has been killed, he talks to Archie and says that he can no longer hear any voices. At one point he says, "God has abandoned me" proving that "The Lamb" was just a part of the sickness controlling his brain.
In The Dead, Matt uses an old British Army Revolver. In The Sacrifice, Matt uses a whip to try and get The Kid's name. Matt carries a homemade banner of The Lamb and The Goat in The Dead and The End.