'Therefore, the true opinion is the fifth, according to which the Pope who is manifestly a heretic ceases by himself to be Pope and head, in the same way as he ceases to be a Christian and a member of the body of the Church; and for this reason he can be judged and punished by the Church. [...]

'The foundation of this argument is that the manifest heretic is not in any way a member of the Church, that is, neither spiritually nor corporally, which signifies that he is not such by internal union nor by external union. For even bad Catholics [i.e. who are not heretics] are united and are members, spiritually by faith, corporally by confession of faith and by participation in the visible sacraments; the occult heretics are united and are members although only by external union; on the contrary, the good catechumens belong to the Church only by an internal union, not by the external; but manifest heretics do not pertain in any manner, as we have already proved.'

St Robert Bellarmine, here.