Psalms 61:2

We should note that this flood of temptations assails the church in three ways: "one is tempted, drawn on and lured by one's own desire," or is worn down by the depravity of false brothers or is assaulted by the more open snares of those outside [the church]. In another place the Lord calls these temptations "the gates of the lower world," and rightly so, for if victorious they drag us down to eternal destruction. He says, "I shall build my church on this rock, and the gates of the lower world shall not prevail against it." Although the gates of the devil strike against it, yet they do not overthrow Christ's church; although the flood of faithlessness inundates it, it does not undermine the house of faith. For [the church] is able to say truthfully to its helper, "When my heart was disquieted, you raised me up on a rock." It is not vanquished by external forces because, by suffering and acquiring the crown of martyrdom, it triumphs over the ferocity of the unbelievers who persecute it. It is not corrupted by false brothers because it refutes the dogmas of heretics by believing properly, and it avoids the vicious example of some Catholics by living soberly and justly and piously. It is not blinded by the smoke of private greed because it is inwardly aflame with the ardor of the Lord's charity alone.

For that Church is founded on a rock, as the Lord says, "Upon this rock I will build my Church." But they build on the sand, as the same Lord says, "Every one that heareth these sayings of mine, and doeth them not, shall be likened unto a foolish man, which built his house upon the sand." But that you may not suppose that the Church which is upon a rock is in one part only of the earth, and does not extend even to its furthest boundaries, hear her voice groaning from the psalm, amid the evils of her pilgrimage. For she says, "From the end of the earth have I cried unto Thee; when my heart was distressed Thou didst lift me up upon the rock; Thou hast led me, Thou, my hope, hast become a tower of courage from the face of the enemy." See how she cries from the end of the earth. She is not therefore in Africa alone, nor only among the Africans, who send a bishop from Africa to Rome to a few Montenses, and into Spain to the house of one lady. See how she is exalted on a rock. All, therefore, are not to be deemed to be in her which build upon the sand, that is, which hear the words of Christ and do them not, even though both among us and among you they have and transmit the sacrament of baptism. See how her hope is in God the Father, the Son, and the Holy Ghost,—not in Peter or in Paul, still less in Donatus or Petilianus.