A Catholic who knowingly and willingly publicly denies a teaching of the Church that must be held with Divine and Catholic Faith, by that very act, is severed from membership in the Catholic Church.
Yes. But...
...there are still details in this principle that are debatable. And that only Church officials, not any priest/laity, can decide.
1. "knowingly and willingly and publically". This can only be determined AFTER THE ACT HAPPENED, not right in the moment. Martin Luther nailed his 99 thesis to the church door knowingly, willingly and publicly, yet the Church still had meetings and gave him a trial.
So none of his "severance" happened immediately. The spiritual severance (based on his own sin and bad will) may have happened immediately (only he and God knows this) but his temporal/govt/priest severance from the Church, by way of excommunication, only happened AFTER church proceedings and a trial.
2. "by that very act". Again, this can only be known later, by human investigation, conversations, etc. +Bellarmine refers to this often, when he talks about the Cardinals rebuking a pope, or judging his actions as heretical.
3. "Severed from membership". There are 2 parts to this - the spiritual severance (i.e. sin of heresy, and the internal will of the heretic to reject the Faith) and the human/govt/office severance, which only happens by way of Church actions - i.e. the human/govt/official acts to remove one from office.