Pope St. Leo the Great (400-461)
"He that sees another in error, and endeavors not to correct it, testifies himself to be in error."

Pope St. Felix III (†492)
"Not to oppose error, is to approve it, and not to defend truth is to suppress it, and indeed to neglect to confound evil men, when we can do it, is no less a sin than to encourage them."

Pope St. Pius V (1504-1572)
"All the evils of the world are due to lukewarm Catholics."

Pope Innocent III
"Not to oppose erroneous Doctrine is to approve of it, and not to defend at all true Doctrine is to suppress it."

St. Athanasius
"God has promised to be like a wall of fire round those who rightly believe in Him."
"Catholics who remain faithful to tradition even if they are reduced to but a handful, they are the True Church of Jesus Christ."
"May God console you!...What saddens you...is the fact that others have occupied the churches by violence, while during this time you are on the outside. It is a fact that they have the premises, but you have the Apostolic Faith. They can occupy our churches, but they are outside the true Faith. You remain outside the places of worship, but the Faith dwells within you. Let us consider: What is more important, the place or the Faith? The true Faith, obviously. Who has lost and who has won in this struggle? The one who keeps the premises or the one who keeps the Faith?"

St. Ambrose (340-397)
"Even heretics appear to possess Christ, for none of them denies the name of Christ. Still, anyone who does not confess everything that pertains to Christ does in fact deny Christ."

St. Thomas Cajetan, O.P. (1469-1534)
Theologian and Cardinal
He points out that the famous axiom "Ubi Petrus, ibi Ecclesia" (Where the Pope is, there is also the Church) holds true only when the Pope acts and behaves as the Pope, because Peter "is subject to the duties of the Office" otherwise, "neither is the Church in him, nor is he in the Church."

Anne Catherine Emmerich (1774-1824)
"The Church is the only one, the Roman Catholic! And if there were left upon earth but one Catholic, he would be the one, universal Church, the Catholic Church, the Church of Jesus Christ against which the gates of Hell shall never prevail."

Cajetan wasn't canonized.

Also, although Anne Emmerich is likely a real saint the writings handed down to us of her visions are wholly kabbalistic and heretical. Best to avoid quoting her supposed words.