"We are the greatest species in existence! And the Emperor is the greatest being of our species! HE IS A MAN, AND YOU WILL PRAY TO HIM AS SUCH!"
Decius XXIII is the Ecclesiarch of the Adeptus Ministorum, and a supporting character in If The Emperor Had a Text-To-Speech Device.
Official Canon
As leader of the Ecclesiarchy, Decius XXIII is one of the most powerful people in the Imperium, as he controls the state religion of worshiping the God-Emperor of Mankind. Decius bitterly hates the Lord Inquisitor Fyodor Karamazov due to an incident that occurred on Salem Proctor. Karamazov executed a young priest who had recently reclaimed his homeworld from heretics, prompting Decius to launch an investigation against him. Karamazov was apparently vindicated, however, when the priests' supporters were found to be heretics themselves. Nonetheless, he hadn't shown any direct proof that his actions had been warranted in the beginning.
TTS Canon
Decius is frequently the butt of the other High Lords jokes, especially thanks to his "stupid taco-shell of a hat." Decius is fervently devoted to the Emperor to the point of religious ecstasy... which is expressed by his hat going erect.
Decius is among the first to hear of the Emperor's decree to disband the Ecclesiarchy and the Inquisition. At first he thinks its a prank from the other High Lords of Terra, but then Karamazov shows up claiming it was the work of a heretic. Decius reluctantly allows Karamazov to search Terra, on the condition that he spilled no blood. Karamzov agrees, only to tell his troops that they should burn any suspects instead.
Upon hearing Karamazov's intentions of storming the Imperial Palace, Decius tries to block him. Karamazov would have killed him, had Kitten not arrived and stated that the order was indeed from the Emperor himself. While Decius and the other High Lords were elated that the Emperor could speak once more, Karamazov refused to listen and swore to get his vengeance.
When Karamazov showed up once again, this time with the full might of the Inquisition, Decius attempted to stop him by having his priests throw furniture at him. When that failed, Decius chased after Karamazov into the throne room, seeing the Emperor himself for the first time. Decius was disappointed to learn from the Emperor that the religion he promoted was a lie created by Lorgar, but was given a new opportunity to help reform the Imperium as a reward for his loyalty. He would help author a new edition of the Lectitio Divinitatus that was more in line with the Emperor's ideals.
Decius was able to convince the rest of the Ecclesiarchy to follow this new creed by claiming that calling the Emperor a God was an insult, and that it would liken him to the inferior Chaos Gods and Xenos dieties. Instead, he was to be worshiped as the Man-Emperor of Mankind.