Final answer:
Coifi's reaction to Paulinus' doctrine was positive; he was ready for a new religion, resulting in a significant moment in Northumbrian conversion to Christianity.
Step-by-step explanation:
The reaction of Coifi, a high priest of the old pagan religion during King Edwin's reign, to the doctrine brought by the missionary Paulinus, is that Coifi is ready for a new religion. In historical accounts, Coifi shows interest in the preaching of Paulinus about Christianity. Seeing the new religion as an opportunity for something more powerful and truthful than the pagan gods who he felt had let him down, Coifi embraces Christianity. His reaction can be seen as a pivotal moment in the conversion of the Northumbrian people, symbolized by Coifi's subsequent actions where he desecrates the pagan altars that he once worshiped.