Thomas Aquinas

Second, he places the duration of the punishment: it shall not be quenched, that is, the effect of this fire will remain forever; smoke, the sign and memory of preceding fire: her smoke ascends for ever and ever (Rev 19:3); and this is to be understood as being while the cause of the threat remains, namely, sin: for thus all prophecies of this sort are to be explained conditionally inasmuch as the Lord himself explains himself: I will suddenly speak against a nation . . . …

Commentary on Isaiah ↗