{The day} (h(8820)h(886d)era). The day of judgment as in  1Th 5:4 
(which see),  Ro 13:12  Heb 10:25 . The work (ergon) of each
will be made manifest. There is no escape from this final
testing. {It is revealed in fire} (en puri apokaluptetai).
Apparently "the day" is the subject of the verb, not the work,
not the Lord. See  2Th 1:8  2:8 . This metaphor of fire was
employed in the O.T. ( Da 7:9f.  Mal 4:1 ) and by John the
Baptist ( Mt 3:12  Lu 3:16f. ). It is a metaphor that must not be
understood as purgatorial, but simple testing (Ellicott) as every
fire tests ({the fire itself will test}, 	o pur auto dokimasei)
the quality of the material used in the building, {of what sort
it is} (hopoion estin), qualitative relative pronoun. Men today
find, alas, that some of the fireproof buildings are not
fireproof when the fire actually comes.