John Gill Commentary Revelation 18:19

"And they cast dust on their heads, and cried, weeping and mourning, saying, Woe, woe, the great city, wherein all that had their ships in the sea were made rich by reason of her costliness! for in one hour is she made desolate." — Revelation 18:19 (ASV)

And they cast dust on their heads
As the seafaring men on account of Tyre, (Ezekiel 27:30) this was a gesture used in mourning when persons were in afflicted and distressed circumstances, denoting disorder, confusion, and debasement; see (Joshua 7:6) (Job 2:12)

and cried, weeping and wailing, saying, alas, alas! that great city ;
as in (Revelation 18:10Revelation 18:16) so it was once, though now in flames:

wherein were made rich all that had ships in the sea ;
not only the merchants of the earth, as in (Revelation 18:3) the cardinals, archbishops, and bishops, but the governors of religious houses; these accumulated great wealth to themselves, and got the best of lands into their possession for the use and support of their abbeys and monasteries:

by reason of her costliness ;
or costly things; pardons, indulgences, absolution, saying Mass, and praying souls out of purgatory, all which are costly, and hereby these traders have been enriched; and now the remembrance of these things, of which they will be deprived, will affect and grieve them, as well as the suddenness of Rome's ruin:

for in one hour is she made desolate ;
her judgment come, and her riches come to nought, (Revelation 18:10Revelation 18:17) .