The Vatican has banished archbishop Carlo Maria Vigano after finding him guilty of schism.
According to BBC, schism is one of the gravest crimes in canon law and occurs when someone withdraws submission to the Pope or his Catholic subjects.
Carlo Maria Vigano served as the Vatican's ambassador to the US from 2011 to 2016 but went into hiding in 2018.
This came after he alleged Pope Francis knew about US cardinal Theodore McCarrick's sexual misconduct and did nothing about it.
The Vatican has rejected this claim.
Vigano also branded the Pope a "false prophet" and a "servant of Satan", before calling for him to resign.
The Vatican's doctrinal office announced the 83-year-old's excommunication - or banishment - on Friday.
It said his previous comments made it clear he refused "to recognise and submit" to the head of the Roman Catholic Church.
Vigano had also rejected the legitimacy of liberal reforms made by the church in the 1960s, it added.
The statement read: "At the conclusion of the penal process, the Most Reverend Carlo Maria Vigano was found guilty of the reserved delict (violation of the law) of schism."
The excommunication means Vigano is formally outside the church and cannot celebrate or receive its sacraments, such as communion
As is normal, the ruling was signed by the head of the Doctrine of the Faith office instead of the Pope himself, but it's highly unlikely the punishment was given without his approval.
Responding on X, where he has frequently posted while in hiding, Vigano remained unrepentant and urged Catholics to support him with a quote from Jesus in the New Testament: "If they keep quiet, the stones themselves will start shouting."
Vigano has cultivated a following of like-minded ultra-conservatives over the years, delving into conspiracy theories and labelling the coronavirus pandemic the "Great Reset".
In a post on X last month, he criticised the Vatican's proceedings against him and accused Pope Francis of representing an "inclusive, immigrationist, eco-sustainable, and gay-friendly" church.