There is much concern over the influence of the Devil in the Church. Indeed, there is a statement from Pope St. Paul VI. “The smoke of Satan has entered the Church.”
How far can Satan get when Church’s own physical members are not spiritual apart of it? This is a question that needs answering.
There are certain protections the Church herself, as the Body of Christ (1 Cor 12:12), protected and guided by the Holy Spirit (John 16:13) is given by the Lord:
A. Infallibility (Dogma declared by the Pope or bishops).
B. Indefectibility (The Church cannot fail to grow or spread the Gospel).
The pope has to declared Ex Cathedra (from his authority as pope) a dogma that “must be adhered to with the obedience of faith” (Lumen Gentium, 24).
These teachings are said to be quite limited in number, only two. Examples: Assumption of Mary, Immaculate Conception.
The Church cannot defect in her mission to spread the Gospel. Part of this is the continued growth of the Church.
While the Diocese of Rome cannot defect, individual dioceses and parishes may. Popes in the past have defected (Pope John XII, Pope Honorius I) in heresy, but not apostasy (total repudiation of the Faith).
Antipopes: There have been antipopes, men who claimed to be pope and were not, who have taught error in particular Arianism (Denial of the Divinity of Christ). Discerning who is pope or not is the first step. He who is pope has the authority.
Mystics like St. Catherine of Siena, were able to detect the true pope, in part through the “test of peace” (Luke 10:5) which resulted in her identifying ever single antipope in her time and also the true pope (Urban VI).
Knowing the pope is knowing the real dogma.
Limits of Corruption
In dogmatic declarations, the spread of the Gospel, the Church is secure. However, in ordinary teaching (Amoris Latitia) there is not guarantee.
Consistent Teaching: One of the safeguards is teaching “since the beginning of the Christian Faith” (Pastor Aeternus, Chapter 4, Section 9).
If the doctrine is novel, and you never heard of it in Scripture, Papal, Christian Patristics (Church Fathers), & then a red light ensues until you are able to clear up the matter. If it contradicts dogma, it is to be immediately rejected and held in contempt.
The above safeguards are given to the Church by Jesus Christ. An “absolutely certain and infallible special revelation” (Council of Trent, Session 6, Canon 16) is an extraordinary gift that God can give (like St.s Catherine of Siena & Teresa of Avila).
Even a pope cannot override such a special revelation, since by it’s nature it is “absolutely certain and infallible.”
This certitude is of faith, and even papal declarations do not have this certitude at the subjective level, but only Infallibility. These special revelations’ quality is, therefore, greater than papal dogma. They trump even the pope, who would never be able to accurately contradict them (Hebrews 7:7).