A valid Pope can never commit the sin of apostasy, or heresy, or schism, because God positively and absolutely prevents it. See my previous article: Can a Pope commit the sin of heresy? By the grace of God, and by the free will decision of the Pope-elect to accept his office, and by the Pope’s continued free will decision to remain in that office (since he could freely resign, if he wished), the Pope is entirely prevented from committing the sins of apostasy, heresy, or schism.
God does not permit this sin in the Pope because it would conflict with the infallibility of the Magisterium and the indefectibility of the Church. The Pope teaches infallibly either alone under Papal Infallibility, or with the body of Bishop in a Council, or with the body of Bishops dispersed through the world (under the ordinary and universal Magisterium). The mere possibility of an heretical Pope would undermine the infallibility of the Magisterium. For the Magisterium never teaches infallibly apart from the teaching of the Pope.
The body of Bishops also can never commit, as a body, the sins of apostasy, or heresy, or schism. Individual Bishops can go astray to any extent. But the body of Bishops united to the Pope can never apostatize from their union with the Pope, nor can they ever, as a body, teach heresy or commit schism. Groups of Bishops can possibly go astray, even committing heresy and schism, as occurred in the eastern schism that formed the Orthodox churches. But the main body of Bishops cannot fall into such sins. God does not permit this to occur because it would conflict with the infallibility of the Magisterium and the indefectibility of the Church. Individual Bishops cannot teach infallibly, but they can teach infallibly as a body united to the Pope, in a Council or by the ordinary and universal Magisterium. The possibility of an heretical or schismatic body of Bishops would undermine the infallibility of the Magisterium and the indefectibility of the Church. And so God absolutely prevents such an event from occurring.
Some ultra-conservative Catholics imagine that the Magisterium is nothing other than the teaching of the Pope. They view the Bishops as merely the assistants of the Pope, as if they are only able to teach under the Magisterium by repeating the teaching of the Popes. Not so. Each Bishop is able to exercise the authentic Magisterium, though only non-infallibly when he is teaching by himself or with a small group of Bishops. But each Bishop participates in the infallible Magisterium by participating in an Ecumenical Council, or by participating in the ordinary and universal Magisterium.
And this brings us to a certain type of error, seen in the position of some ultra-conservative heretics. They have decided that the main body of Bishops, not all Bishops but supposedly the vast majority, have gone astray from the true Faith. This heretical claim is seen in groups such as the SSPX and various sedevacantist splinter groups, who imagine that all the Popes since Vatican II, and the body of Bishops with them, have all committed heresy. But the same error, in a lesser form, is seen among some Catholics, who believe that every Pope has remained faithful, but who nevertheless think that most Bishops have gone astray.
An example of this latter type of error is found in Michael Voris’ videos, which are filled with rants against the Bishops as a body, and repeated claims that they have almost all gone astray. In his video on contraception, he claims that the Bishops on the whole have rejected the teaching of Humanae Vitae, by ignoring it or by actively undermining it. If this claim were true, it would mean that the Bishops as a body had committed formal heresy, because the teaching on contraception is infallible, and even obstinate doubt about an infallible magisterial teaching is heresy. Fortunately, it is not true.
The teaching of the Church on contraception is infallible because it falls under the ordinary and universal Magisterium:
“Although the individual bishops do not enjoy the prerogative of infallibility, they nevertheless proclaim Christ’s doctrine infallibly whenever, even though dispersed through the world, but still maintaining the bond of communion among themselves and with the successor of Peter, and authentically teaching matters of faith and morals, they are in agreement on one position as definitively to be held.” (Second Vatican Council, Lumen Gentium, n. 25.
The Bishops dispersed through the world have consistently taught, along with successive Popes, that the use of contraception is intrinsically evil and always gravely immoral. The teaching of the Church on contraception could not possibly fall under the infallibility of the ordinary and universal Magisterium without this constant teaching of the body of Bishops. So obviously they are not in error on this topic.
Instead, it is Michael Voris who is teaching a series of heresies, along with the false claim that his teachings are that of the Catholic Church. More on Michael Voris’ errors here.