A.T. Robertson Commentary Revelation 5:2

"And I saw a strong angel proclaiming with a great voice, Who is worthy to open the book, and to loose the seals thereof?" — Revelation 5:2 (ASV)

A strong angel (αγγελον ισχυρον). One needed (10:1; 18:21) "whose call could reach to the farthest limits of the universe" (Beckwith) and so "with a great voice" (εν φωνη μεγαλη, in a great voice, as in 14:7,9,15, and without εν 5:12; 6:10; 7:2,10; 8:13; 10:3, etc.). See εν ισχυρα φωνη (18:2).

Proclaiming (κηρυσσοντα). Present active predicate participle of κηρυσσω, to herald, to preach.

Worthy to open and to loose (αξιος ανοιξα κα λυσα). Worthy by rank and character (cf. John 1:27) as well as by ability (εδυνατο, verse 3), followed by two infinitives (first aorist active) of ανοιγω and λυω, though ινα and the subjunctive can be used after αξιος as in Joh 1:27. Here αξιος is like ικανος (capable, qualified) as in Mt 8:8. The articles here (το, τας) refer to the book and the seals in verse 1. It is a husteron-proteron, since the loosing of the seals precedes the opening of the book.