First Vatican Council, Session 3, Chapter 3: “Wherefore, by divine and catholic faith all those things are to be believed which are contained in the word of God as found in scripture and tradition, and which are proposed by the church as matters to be believed as divinely revealed, whether by her solemn judgment or in her ordinary and universal magisterium.”
Pope Leo XIII, Satis Cognitum: “#9 The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium…Christ instituted in the Church a living, authoritative and permanent Magisterium, which by His own power He strengthened, by the Spirit of truth He taught, and by miracles confirmed. He willed and ordered, under the gravest penalties, that its teachings should be received as if they were His own. As often, therefore, as it is declared on the authority of this teaching that this or that is contained in the deposit of divine revelation, it must be believed by every one as true. If it could in any way be false, an evident contradiction follows; for then God Himself would be the author of error in man.
Pope St. Pius X, The Oath against the Errors of Modernism: “I . . . firmly embrace and accept all and everything that has been defined, affirmed, and declared by the unerring magisterium of the Church, especially those chief doctrines which are directly opposed to the errors of this time.
Pope Pius XI, Divini Illius Magistri: “(16) Upon this magisterial office Christ conferred infallibility, together with the command to teach His doctrine…(18)Hence it is that in this proper object of her mission, that is, “in faith and morals, God Himself has made the Church sharer in the divine magisterium and, by a special privilege, granted her immunity from error“.
My Comment: There are those who believe that the Magisterium of the Church can err in certain situations. There are others that believe that there is a “type” or “mode” etc… of the Magisterium that is not protected from Infallibility. Neither of these opinions are true. It is true that Theologians have dubbed certain teachings by the Popes as non-Infallible but this is just a poor choice of words to use “magisterium”. Everyone should know that not everything the Pope says is infallible, it just means that those teachings are not part of the Magisterium.
What is clear though, is that whenever the Church Magisterium speaks of itself and its authority, it always describes itself as unable to err and attributes this to the promises of our Lord. One could never find a quote by the Church teaching that there’s a mode of the Magisterium which is not Infallible.