Trinity at all costs
"... but whoever blasphemes against the Holy Spirit will never be forgiven; they are guilty of an eternal sin." Mark 3.29
Arianism refers to the heretical belief that Jesus was not of the same substance as God and was therefore subordinate to him. The rot set in when some early believers misunderstood the nature of the Holy Spirit as being something that merely emanated from God, as opposed to being one of the persons in whom God is manifest. The consequence of these misapprehensions is that they undermined orthodox Christian convictions about the Holy Trinity - in which Father, Son and Holy Spirit exist together as one God in three persons - and paved the way for Islam, Unitarianism and Mormonism. Surely that was why Jesus was so protective of the status of the Holy Spirit?