The Spirit of truth enables her to do this: “When the Paraclete comes, whom I shall send you from the Father, the Spirit of truth, who proceeds from the Father, he shall give testimony of me. And you shall give testimony because you are with me from the beginning” (John 15:26.)
The continuous presence of Christ keeps his Church from all error: “Going, therefore, teach all nations . . . teaching them to observe all things whatsoever I have commanded you; and behold, I am with you all days even to the consummation of the world” (Matt. 19:20).
Certainly, God could not command, under pain of eternal damnation, the acceptance of truth on fallible human authority. He could not say of truth taught by man unaided, “He that believes and is baptized shall be saved, but he that believes not shall be condemned” (Mark 16:16) Yet that is what he said about his Church’s teaching.