Who is this King of glory? - See the notes at Psalm 24:8.
The Lord of hosts, he is the King of glory - On the meaning of the phrase, "the Lord of hosts," see the notes at Isaiah 1:9. The essential idea is, that God rules over the universe of worlds considered as marshalled in order, or arrayed as hosts or armies are for battle. All are under His command. The stars in the sky, that seem to be marshalled and led forth in such perfect and beautiful order - the inhabitants of heaven in their different orders and ranks - all these acknowledge Him, and submit to Him as the supreme God. In the close of the psalm, therefore, there is an exact accordance with the thought in the beginning, that God is the Sovereign Ruler of the universe, and that He should everywhere be recognized and regarded as such. The entrance of the ark of the covenant into the place provided for it as a permanent residence was a fit occasion to proclaim this thought; and this is proclaimed in the psalm in a manner befitting so solemn an occasion and so sublime a truth.

Who is this King of glory?.... This is repeated, because of the preceding words, and in order to have a further account of his glorious Person, as follows:
the Lord of hosts, he is the King of glory; he who is the Lord of sabaoth, the Lord of the armies, both of the heavens and the earth; at whose dispose and control all things are in both worlds, above and below: this is the great and glorious Person that condescends to dwell in his churches, and in the hearts of his people; and this honour have his saints.
Selah; on this word; see Gill on Psalm 3:2.

Lord of hosts--or fully, Lord God of hosts (Hosea 12:5; Amos 4:13), describes God by a title indicative of supremacy over all creatures, and especially the heavenly armies (Joshua 5:14; 1-Kings 22:19). Whether, as some think, the actual enlargement of the ancient gates of Jerusalem be the basis of the figure, the effect of the whole is to impress us with a conception of the matchless majesty of God.

Of hosts - Under whose command are all the hosts of heaven and earth, angels and men, and all other creatures.