Official Canon Edit
Lorgar was found on Colchis, a desert planet that was ruled by a theocracy of proto-Chaos worshipers. Dubbed the Old Faith, the priests sought to slay Lorgar when he started receiving visions of the Emperor's arrival. Instead, Lorgar lead an uprising against the priests, and took control of the planet. When the Emperor found him, he was given command of the Word Bearers legion.
During the Great Crusade, Lorgar spent much time turning the worlds he conquered into centers of spitirual enlightenment. In secret, he had written the Lectitio Divinitatus, declaring the Emperor to be a God, in direct defiance to the Imperial Truth. Upon learning this, the Emperor was furious, and had the Ultramarines destroy the Word Bearers' capital, while he psychically forced Lorgar and his legion to kneel to himself, Roboute Guilliman, and Malcador.
Horus Heresy Edit
Humiliated, Lorgar could no longer look up to his father. His First Captains and Head Chaplain, Kor Phaeron and Erebus, told Lorgar that the Old Faith was real, and that he should seek out the Gods of Chaos. Thus Lorgar was the first Primarch to join the Ruinous Powers, and set about corrupting his brothers.
As the Heresy began, Lorgar got his revenge upon the Ultramarines, proving how brutal a warlord he could be as he readily embraced the use of daemons and daemonic possession to enhance his legion's strength. His previously restrained psychic powers also grew, making him a potent Sorcerer.
After becoming a Daemon Prince of Chaos Undivided, Lorgar has done very little except teach Abbadon how to use daemons. In an ironic turn of events, his book the Lectitio Divinitatus became the foundation of the state religion of the Imperium, much to his eternal regret.
TTS Canon Edit
The Emperor complained that Lorgar was a brainwashed choir boy who couldn't use his brain to realize that he was being manipulated by his Chaos-worshiping subordinates Erebus and Kor Phareon. He also claimed that the incident that broke Lorgar's loyalty was just to set him straight and get him to "stop doing stupid shit," after which Lorgar betrayed him for seemingly no reason.