Even though the quality of the foundation was the best, the condition of the building also depended on what others built on top of the foundation. In Paul"s day contractors built buildings of durable and or combustible materials, as they do today. In the building of the Corinthian church durable materials were those activities that sprang from reliance on Christ and Him crucified, the foundation. These works contributed to the permanent spiritual strengthening of the believers. The combustible materials were activities that arose out of human "wisdom" in all its forms. These made no lasting contribution though they may have served some temporary need. Examples of the former include instruction in the Word of God, training in evangelism, and the refutation of error. Illustrations of the latter would be the teaching of popular ideas not rooted in Scripture, social work that excluded the gospel message, and the use of time and money for simply temporal purposes. However, Paul"s main concern in this metaphor was those doing the building rather than the building itself. [source][source][source]
"The six materials in 1 Corinthians 3:12 are arranged to denote a descending scale by moving from a unit of three good qualities to a unit of three bad ones. The verse uses pictures to represent what Paul calls "work" in 1 Corinthians 3:13-14. Paul"s main point is to encourage building with quality materials that will meet with God"s approval and receive eternal reward. Interpreters sometimes restrict the meaning of the symbols either to doctrine, to people, to activity, or to character. The [1] conclusion is that Paul in the symbols combines several things that lead to Christ"s good pleasure and a believer"s reward. These are sound doctrine, activity, motives and character in Christian service." [2][source]
God will expose the work of each of God"s servants on "the day." This is a reference to the day when the believer will stand before God and give an account of the stewardship of his or her life at Christ"s judgment seat (cf. Luke 19:11-27; 1 Corinthians 1:8; 2 Corinthians 5:10; Philippians 1:6; Philippians 1:10; 2 Timothy 1:12; 2 Timothy 1:18; 2 Timothy 4:8; Revelation 22:12; et al.). [3] Then the fire of God"s judgment will test the quality of each person"s work and his workmanship, but not his person. The durability or transience of those works will then become apparent. [source][source][source]