Adam Clarke on Revelation 9:20

Yet repented not - The commission which these horsemen had was against idolaters; and though multitudes of them were destroyed, yet the residue continued their senseless attachment to dumb idols, and therefore heavier judgments might be expected. These things are supposed to refer to the desolation brought upon the Greek Church by the Ottomans, who entirely ruined that Church and the Greek empire. The Church which was then remaining was the Latin or western Church, which was not at all corrected by the judgments which fell upon the eastern Church, but continued its senseless adoration of angels, saints, relics, etc., and does so to the present day. If, therefore, God's wrath be kindled against such, this Church has much to fear.