Augustine (354-430): God alone swears securely, because He alone is infallible. In Psalmum LXXXVIII Enarratio, Sermo I, PL 37:1122
Augustine (354-430) written 413/414 AD: Do not doubt that this duty of ours [i.e., to intercede for others] is a part of religion because God, ‘with whom there is no iniquity,’ whose power is supreme, who not only sees what each one is but also foresees what he will be, who alone cannot err in His judgment because He cannot be deceived in His knowledge, nevertheless acts as the Gospel expresses it: ‘He maketh his sun to rise upon the good and bad, and raineth upon the just and the unjust.’ See In Epistola CLIII, Caput II, §4, PL 33:654.
Augustine (354-430): All such matters, therefore, being put out of sight, let them show their Church, if they can; not in the discourses and reports of Africans, not in the councils of their own bishops, not in the writings of any controversialists, not in fallacious signs and miracles, for even against these we are rendered by the word of the Lord prepared and cautious, but in the ordinances of the Law, in the predictions of the Prophets, in the songs of the Psalms, in the words of the very Shepherd himself, in the preachings and labours of the Evangelists, that is, in all the canonical authorities of sacred books. Nor so as to collect together and rehearse those things that are spoken obscurely, or ambiguously, or figuratively, such as each can interpret as he likes, according to his own views. For such testimonies cannot be rightly understood and expounded, unless those things that are most clearly spoken are first held by a firm faith. De Unitate Ecclesiae, Caput XVIII, §47, PL 43:427-428.
Augustine (354-430): We ought to find the Church, as the Head of the Church, in the Holy Canonical Scriptures, not to inquire for it in the various reports, and opinions, and deeds, and words, and visions of men. De Unitate Ecclesiae, Caput XIX, §49, PL 43:429.
Augustine (354-430): Whether they [i.e. the Donatists] hold the Church, they must show by the Canonical books of the Divine Scriptures alone; for we do not say, that we must be believed because we are in the Church of Christ, because Optatus of Milevi, or Ambrose of Milan, or innumerable other bishops of our communion, commended that Church to which we belong, or because it is extolled by the Councils of our colleagues, or because through the whole world in the holy places which those of our communion frequent such wonderful answers to prayers or cures happen. De Unitate Ecclesiae, Caput XIX, §50, PL 43:429-430.