Will this purification be by fire?
Obj. 1 - But fire is a part of the of the world so fire itself must be purified.
Resp. 1 - Fire is not used by us now in its own matter (pure state) and thus matter will be able to be purified by fire in its pure state
Obj. 2 - But does not water also has a purifying power (deluge)
Resp. 2 - The Deluge was a suitable purification for the personal infection of sin, but for the impurity mixed in with creatures water has not power to purify/separate. (fire can purify all things with or without the destruction)
Obj. 3 - By saying cleansing, this means to separate, but only God can do this, not a creature.
Resp. 3 – This is a purgation which means to return things to the purity in which they were created which can be done through an instrument, namely fire.
To all these we read in Scripture:
II Pet 3,10 “the day of the Lord will come as a thief; at that time the heavens will pass away with great violence and the elements will be dissolved with heat and the earth and the works that are in it will be burned up”
Purification by fire in convenient for 3 reasons :
i) fire is the most noble of four elements is has properties most similar to glory (light)
ii) fire is not contaminated from something outside since it is an active power
iii) fire is not common (not an element) that can become tainted by sin and thus has a great efficacy to purify