"Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is. If any man's work abide which he hath built thereupon, he shall receive a reward."
The first thing to note about these verses is that the apostle is not talking about the eternal reward of the Christian, but rather the reward for service rendered in the ministry. The entire chapter is about methods of planting and building in the establishment of believers and churches. Paul describes his approach and also that of his fellow ministers.
So we can immediately discount any idea of predestination or of salvation regardless of one's works. It seems quite clear that Paul was describing an actual situation in which some weak pastors or teachers, after bringing people to Christ, had not followed through with sound instructions about the Christian life. As a result, there had not been satisfactory spiritual growth of the converts. Paul spoke of such inferior teaching as poor building material and indicated that it would be destroyed in the fiery test. The literal fires of the last day are not being referred to here, for the fire represents a testing work, and those who pass through it may be saved (verse 15).
Paul is conveying the sobering thought that even though a minister may repent of his poor workmanship and be saved himself, the results of his faulty work (weak instruction) cannot be changed. Souls could be lost by his poor building material when they faced severe spiritual stress. Nevertheless, the worker could be saved, because at least he laid the proper foundation in presenting Christ as the basis of salvation.