Oecumenius

He says that all have been defeated and fallen under his feet, and there was given unto him a mouth speaking great things and blasphemy. Was it given by someone? It was given by those deceived and having worshiped him; for to boast arrogantly is pride. For what is greater than this? To say, "I will ascend into heaven; I will set my throne above the stars of heaven." (Isa. 14:13) And just a little after that, he says, "I will be like the Most High," (Isa. 14:14) as Isaiah mocks him, for these are indeed blasphemies against God. …

Commentary on Revelation ↗

Dionysius of Alexandria

And in like manner it is revealed to John; "For there was given to him," he says, "a mouth speaking great things and blasphemy; and there was given unto him authority and forty and two months." It is wonderful that both of these things occurred under Valerian; and it is the more remarkable in this case when we consider his previous conduct, for he had been mild and friendly toward the men of God, for none of the emperors before him had treated them so kindly and favorably.

Epistle to Hermammon (preserved in Eusebius, Church History 7.10) ↗

Bede

And he was given authority to act for forty-two months, etc. For the first three and a half years, he does not openly blaspheme but acts in the mystery of iniquity, which, after the dissension and the revelation of the man of sin, will be exposed. Then he will say: I am Christ (Matthew XXIV); now, however: Behold, here is Christ, and there (Ibid.). By against God, it signifies against God.

Commentary on Revelation ↗