Jamieson, Fausset and Brown on 1 Corinthians 3:12

12. Now--rather, "But." The image is that of a building on a solid foundation, and partly composed of durable and precious, partly of perishable, materials. The "gold, silver, precious stones," which all can withstand fire (Re 21:18, 19), are teachings that will stand the fiery test of judgment; "wood, hay, stubble," are those which cannot stand it; not positive heresy, for that would destroy the foundation, but teaching mixed up with human philosophy and Judaism, curious rather than useful. Besides the teachings, the superstructure represents also the persons cemented to the Church by them, the reality of whose conversion, through the teachers' instrumentality, will be tested at the last day. Where there is the least grain of real gold of faith, it shall never be lost (1Pe 1:7; compare 1Co 4:12). On the other hand, the lightest straw feeds the fire [Bengel] (Mt 5:19).