Bellarmine: “A general Council represents the universal Church, and hence has the consensus of the universal Church; therefore, if the Church cannot err, neither can a legitimate and approved Ecumenical Council err.”

However, this only applies to what is approved by the Roman Pontiff. And again, in Saint Robert Bellarmine, De Controversiis, there are several chapters explaining from Scripture, Tradition, and from reason that Ecumenical Councils, approved by the Roman Pontiff, cannot err on faith or morals.

Bellarmine: “It must be held with Catholic faith that general Councils confirmed by the Supreme Pontiff can neither err in faith nor morals.”

The same idea is expressed by Ludwig Ott, in his famous book The Fundamentals of Catholic Dogma:

“It has been the constant teaching of the Catholic Church from the earliest times that the teachings of the General Councils are infallible.”

In his letter to archbishop Lefebvre, Pope Saint Paul VI stated that some teachings of Vatican II are definitive on matters of faith and require the full assent of faith, implying that these teachings are infallible. Not only definitions of Councils are infallible, but also anything on faith or morals taught definitively by the Council, approved by the Pope.

This is still something of an open question, as some persons hold that Councils can teach non-infallibly. But it is dogma at least that the non-infallible teachings of Popes and Councils cannot err gravely on doctrine or discipline.

So the decision of Pope Francis on the TLM is not a grave error, and is certainly the decision of Christ, not merely a human decision. For Christ is always with His Vicar, guiding His Church.