I answer that, Some have understood the "foundation" to be dead faith, upon which some build good works, signified by gold, silver, and precious stones, while others build mortal sins, which according to them are designated by wood, hay and stubble. But Augustine disapproves of this explanation (De Fide et Oper. xv), because, as the Apostle says (Gal. 5:21), he who does the works of the flesh, "shall not obtain the kingdom of God," which signifies to be saved; whereas the Apostle says that he who builds wood, hay, and stubble "shall be saved yet so as by fire." Consequently wood, hay, stubble cannot be understood to denote mortal sins.
Others say that wood, hay, stubble designate good works, which are indeed built upon the spiritual edifice, but are mixed with venial sins: as, when a man is charged with the care of a family, which is a good thing, excessive love of his wife or of his children or of his possessions insinuates itself into his life, under God however, so that, to wit, for the sake of these things he would be unwilling to do anything in opposition to God. But neither does this seem to be reasonable. For it is evident that all good works are referred to the love of God, and one's neighbor, wherefore they are designated by "gold," "silver," and "precious stones," and consequently not by "wood," "hay," and "stubble."
We must therefore say that the very venial sins that insinuate themselves into those who have a care for earthly things, are designated by wood, hay, and stubble. For just as these are stored in a house, without belonging to the substance of the house, and can be burnt, while the house is saved, so also venial sins are multiplied in a man, while the spiritual edifice remains, and for them, man suffers fire, either of temporal trials in this life, or of purgatory after this life, and yet he is saved for ever.