And I saw a strong angel - An angel endowed with great strength, as if such strength was necessary to enable him to give utterance to the loud voice of the inquiry. "Homer represents his heralds as powerful, robust men, in order consistently to attribute to them deep-toned and powerful voices" (Prof. Stuart). The inquiry to be made was one of vast importance; it was to be made of all in heaven, all on the earth, and all under the earth, and hence an angel is introduced so mighty that his voice could be heard in all those distant worlds.
Proclaiming with a loud voice - That is, as a herald or crier. He is rather introduced here as appointed to this office than as self-moved. The design undoubtedly is to impress the mind with a sense of the importance of the disclosures about to be made, and at the same time with a sense of the impossibility of penetrating the future by any created power. That one of the highest angels should make such a proclamation would sufficiently show its importance; that such an one, by the mere act of making such a proclamation, should practically confess his own inability, and consequently the inability of all of similar rank, to make the disclosures, would show that the revelations of the future were beyond mere created power.
Who is worthy to open the book, - That is, who is "worthy" in the sense of having a rank so exalted, and attributes so comprehensive, as to authorize and enable him to do it. In other words, who has the requisite endowments of all kinds to enable him to do it? It would require moral qualities of an exalted character to justify him in approaching the seat of the holy God, to take the book from his hands; it would require an ability beyond that of any created being to penetrate the future, and disclose the meaning of the symbols which were employed. The fact that the book was held in the hand of him that was on the throne, and sealed in this manner, was in itself a sufficient proof that it was not his purpose to make the disclosure directly, and the natural inquiry arose whether there was anyone in the wide universe who, by rank, or character, or office, would be empowered to open the mysterious volume.

A strong angel - One of the chief of the angelic host.
Proclaiming - As the herald of God.
To open the book, and to loose the seals - To loose the seals that he may open the book. Who can tell what this book contains? Who can open its mysteries? The book may mean the purposes and designs of God relative to his government of the world and the Church; but we, whose habitation is in the dust, know nothing of such things. We are, however, determined to guess.

And I saw a strong angel proclaiming with a loud voice,.... This was not the uncreated angel, Jesus Christ; for he is after spoken of as the lion of the tribe of Judah, and as the Lamb in the midst of the throne, but a created one; though who he was, whether Gabriel, as some say, because his name signifies the strong or mighty One of God, is not material to be known; angels are said to be mighty, and to excel in strength: this is called so here, chiefly with respect to his voice, which he, by reason of his great strength, exerted so loudly, as to be heard by all the creatures in heaven, and in earth, and under the earth however, this was not John the Baptist, but if a minister of the Gospel, rather some one since, making the following proclamation:
who is worthy to open the book, and to loose the seals thereof? suggesting, that if there was any such person, that he was desired to come, and do it; and it seems, that he must not only be one of power and ability, but of dignity and authority, a person of greatness and worth, from whence his ability arises; as he ought to be that can engage his heart to draw nigh to God, and take a book out of his right hand, this mysterious book of the Revelation, as it was indeed while sealed; and open and explain it to others, unseal it or exhibit it, as it afterwards was in the following scenes and visions, and gave a view of all that is contained in it; and not only so, but fulfil and accomplish all the prophecies in it; and who can, or is worthy to do all this, but he that sits at the right hand of God, and who is God as well as man? and this proclamation was made to stir up an earnest desire in John, and all the saints he represents, to know what was in this book; and to show the impotence of all creatures to make any discovery of it; and to illustrate, and set off with a greater foil, the glory, excellency, ability, and worth of Jesus Christ. There seems to be an allusion to the president of the temple calling to the priests under him, to attend to the several parts of service assigned them, saying unto them, , "whosoever is worthy, let him" do so and so (c).
(c) Misna Tamid, c. 1. sect. 4. & c. 5. sect. 4. 5.

strong-- (Psalm 103:20). His voice penetrated heaven, earth, and Hades (Revelation 10:1-3).

And I saw a strong angel - This proclamation to every creature was too great for a man to make, and yet not becoming the Lamb himself. It was therefore made by an angel, and one of uncommon eminence.