Even the demons cried out, on beholding the Son: “I know who You are, the Holy One of God.” Later the devil looking at him and tempting him, would say: “If You are the Son of God.” All of these thus recognized the Son and the Father, yet without believing. So it was fitting that the truth should receive testimony from all, and should become a means of judgment for the salvation not only of those who believe, but also for the condemnation of those who do not believe. The result is that all should be fairly judged, and that the faith in the Father and Son should be a matter of decision for all, so that one means of salvation should be established for all, receiving testimony from all, both from those belonging to it who were its friends, and by those having no connection with it who were its enemies. For that evidence is most trustworthy and true which elicits even from its adversaries striking testimonies on its behalf.
Irenaeus, Against Heresies 4.6.6–7
And again, when He put a curb in the mouths of the demons that cried after Him from the tombs. For although what they said was true, and they lied not then, saying, ‘You are the Son of God,’ and ‘the Holy One of God;’ yet He would not that the truth should proceed from an unclean mouth, and especially from such as them, lest under pretense thereof they should mingle with it their own malicious devices, and sow these also while men slept. Therefore He suffered them not to speak such words, neither would He have us to suffer such, but hath charged us by His own mouth, saying, ‘Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves;’ and by the mouth of His Holy Apostles, ‘Believe not every spirit.’ Such is the method of our adversary’s operations; and of the like nature are all these inventions of heresies, each of which has for the father of its own device the devil, who changed and became a murderer and a liar from the beginning. But being ashamed to profess his hateful name, they usurp the glorious Name of our Savior ‘which is above every name,’ and deck themselves out in the language of Scripture, speaking indeed the words, but stealing away the true meaning thereof; and so disguising by some artifice their false inventions, they also become the murderers of those whom they have led astray.
Athanasius, To the Bishops of Egypt 1.3