The damning verdict came from Cardinal Ivan Dias, prefect of the Congregation for the Evangelisation of Peoples, who is the most senior Catholic delegate invited to the Lambeth Conference - the once-a-decade gathering of the world's Anglican bishops.
In his address, Dias was careful not to single out the Anglican church, which has been afflicted by divisions since the consecration of a gay bishop in 2003.
Instead he referred to "Christian communities".
However, his unequivocal language laid bare his disapproval of the chaos sweeping through the world's third biggest Christian denomination. He said: "When we live myopically in the fleeting present, oblivious of our past heritage and apostolic traditions, we could well be suffering from spiritual Alzheimer's.
"When we behave in a disorderly manner, going whimsically our own way without any coordination with the head of or the other members of our community, it could be ecclesial Parkinson's."
Dias is highly regarded in Rome and was one of the cardinals considered papabile - worthy of being elected pope - at the 2005 conclave that eventually selected Benedict XVI.
His speech is the latest in a series of criticisms from Rome about the liberal drift of the Anglican church.
Earlier this month, the Vatican expressed regret that the Church of England's General Synod voted to proceed with the ordination of women bishops, while on the eve of the conference the second in command at the Holy See, Tarcisio Cardinal Bertone, warned that the crises gripping Anglicanism posed a "further and grave challenge" to the relationship between the two denominations.
Dias told bishops the battle to bring Christ to the world must be placed in the "wider context of spiritual combat" with Satan. "If this context is ignored in favour of a myopic world-vision, Christ's salvation will be conveniently dismissed as irrelevant."
This "spiritual warfare" had continued since the fall of Adam, raging "aided and abetted by well-known secret sects, Satanic groups and New Age movements" that revealed the "many ugly heads of the hideous anti-God monster".
These works of the devil were, he added, "secularism, which seeks to build a godless society; spiritual indifference, which is insensitive to transcendental values; and relativism, which is contrary to the permanent tenets of the Gospel".
"We Christians and bishops can ill afford to remain on the sidelines as passive spectators," he warned.
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