Job 18:5

The rest of Bildad's discourse is entirely taken up in an elegant description of the miserable condition of a wicked man, in which there is a great deal of certain truth, and which will be of excellent use if duly considered - that a sinful condition is a sad condition, and that iniquity will be men's ruin if they do not repent of it. But it is not true that all wicked people are visibly and openly made thus miserable in this world; nor is it true that all who are brought into great distress and trouble in this world are therefore to be deemed and adjudged wicked men, when no other proof appears against them; and therefore, though Bildad thought the application of it to Job was easy, yet it was not safe nor just. In these verses we have, I. The destruction of the wicked foreseen and foretold, under the similitude of darkness (Job 18:5, Job 18:6): Yea, the light of the wicked shall be put out. Even his light, the best and brightest part of him, shall be put out; even that which he rejoiced in shall fail him. Or the yea may refer to Job's complaints of the great distress he was in and the darkness he should shortly make his bed in.

— Matthew Henry's Concise Commentary (public domain)

That (Job 18:4) cannot be. The decree of God is unalterable, the light (prosperity) of the wicked shall at length be put out. his fire--alluding to Arabian hospitality, which prided itself on welcoming the stranger to the fire in the tent, and even lit fires to direct him to it. The ungodly shall be deprived of the means to show hospitality. His dwelling shall be dark and desolate!

— Jamieson-Fausset-Brown Commentary (public domain)