Revelation 16:8

But these things are not difficult to explain in terms of the turns of fortune. For the scorching of the sun would be to people the wasting and the crowding and the destitution of those left in wars, who, oppressed by evils, ought to have asked from the Almighty God aid and deliverance from the afflictions that hold them.

Perhaps humankind will literally be burned by the fierce heat of the sun. While God in his goodness "curbs them with bit and bridle who do not draw near to him" so that they might look toward repentance, some have fallen to such depths of evil that by the depravity of their minds they will not turn to conversion but rather to blasphemy. Perhaps, however, the image of the sun signifies the course of the day that burns those worthy of scourging by the heat of temptations, so that by the experience of sufferings they might learn to hate the mother of these things, namely, sin. However, those who are mindless concerning any knowledge of their own faults will "sharpen their tongue" against God, just as even now we see many who are distressed by the unspeakable horrors that encompass us at the hands of the barbarians and who blame the goodness of God, since he has held such evils for our own generation.

And the fourth poured out his bowl on the sun, etc. The persecutors of the Church, who, like a burning sun, try to dry up the seed of God's word, are destined to be burned with the future fire of hell. Or if you interpret the sun as the splendor of the wise, it is not the angel pouring out on the sun, but the sun itself given the power to affect men with heat and fire, because when wise men, overcome by tortures, are touched by the error of evil deeds, persuaded by their example, the weak burn with temporal desires. However, the heat, as we have said, can also be understood as the body of the devil being irremediably tormented by the stability of the saints and being incited to blasphemy. Of which the prophet says: Zeal, he says, has taken hold of an uneducated people; and now fire consumes the adversaries. And now, indeed, except for the final fire of judgment.