The message of the competing religion contained more than denying the Atoning Messiah. There could not be that many “flattering words” that would deny the Messiah, retain everything else, and still lead people to “commit many sins.” This alternate worldview would certainly lead to different practices, which could easily be seen as sin by orthodox believers, requiring them to be “admonished by the church.”
The shift to a more Mesoamerican worldview would bring with it a different understanding of God’s nature and how the world works and, hence, a significant difference in the doctrine of sacrifices. While the practice of sacrifices would remain, the reason for the offering and the being to whom it was offered would all change. For a monotheistic people, offering sacrifices to a different god was clearly sin.