That the Persecution originated on Account of the Oracle of Apollo, who, it was said, could not give Oracles because of the Righteous Men
About that time it is said that Apollo spoke from a deep and gloomy cavern, and through the medium of no human voice, and declared that the righteous men on earth were a bar to his speaking the truth, and accordingly that the oracles from the tripod were fallacious. Hence it was that he suffered his tresses to droop in token of grief, and mourned the evils which the loss of the oracular spirit would entail on mankind. But let us mark the consequences of this.
Source: Life of Constantine (New Advent)