At that time, though the Emperor was dissatisfied with Lorgar's behavior, he could still accept it. He merely thought the child had been led astray by the planet's unhealthy customs. Once he was brought home, all it would take was teaching him more about the Imperial Truth. After all, as a Primarch, Lorgar should have been exceptionally intelligent and able to understand the Emperor's intentions at once.
In the end, the Emperor had overestimated Lorgar's emotional intelligence and intellect. Lorgar treated the Imperial Truth sent by the Emperor as waste paper and wholeheartedly promoted religious theories throughout the Imperium of the Great Crusade and among his brothers.
But as mentioned before, Lorgar was not truly devout by nature. His weakness made him desperately crave something to rely on.
When Lorgar was young, Kor Phaeron had been his support. After he grew up, the Emperor became his support.
He sought a powerful being, hoping to become that being's servant and receive its protection and guidance, just as he had interacted with Kor Phaeron in his youth.
On top of that, the Emperor himself had terrible taste and emotional intelligence. During the Great Crusade, every word and deed of his was practically indistinguishable from those of a god. He wore gleaming golden armor all day, his majestic, radiant face impossible to look upon directly. This led Lorgar to make the wrong judgment.
The Emperor was actually a tsundere. He said he was not a god, yet his daily behavior already hinted to everyone: Look how much I resemble a god. You should sincerely respect yourselves.
And in order to spread the idea that the Emperor was a god more effectively and quickly, Lorgar used his earth-shattering Primarch intellect to write the Emperor's Litany. With rigorous thought and clear logic, it demonstrated why