A.T. Robertson Commentary Revelation 13:5

"and there was given to him a mouth speaking great things and blasphemies; and there was given to him authority to continue forty and two months." — Revelation 13:5 (ASV)

There was given to him (εδοθη αυτω). First aorist passive indicative of διδωμ, to give, as in next line and verse 7. Perhaps a reference to εδωκεν (he gave) in verse 4, where the dragon (Satan) gave the beast his power. The ultimate source of power is God, but the reference seems to be Satan here.

Speaking great things and blasphemies (λαλουν μεγαλα κα βλασφημιας). Present active participle of λαλεω, agreeing with στομα (nominative neuter singular and subject of εδοθη). The words are like Daniel's description of the Little Horn (7:8,20,25) and like the description of Antiochus Epiphanes (I Macc. 1:24). Cf. 2 Peter 2:11.

To continue (ποιησα). First aorist active infinitive (epexegetic use) of ποιεω, either in the sense of working (signs), as in Da 8:12-14, with the accusative of duration of time (μηνας months), or more likely in the sense of doing time, with μηνας as the direct object as in Mt 20:12; Acts 20:3; James 4:13.