Via Ronald Knox, 1 Cor 3.10-15:
10With what grace God has bestowed on me, I have laid a foundation as a careful architect should; it is left for someone else to build upon it. Only, whoever builds on it must be careful how he builds.11The foundation which has been laid is the only one which anybody can lay; I mean Jesus Christ.12But on this foundation different men will build in gold, silver, precious stones, wood, grass, or straw,13and each man’s workmanship will be plainly seen. It is the day of the Lord that will disclose it, since that day is to reveal itself in fire, and fire will test the quality of each man’s workmanship.14He will receive a reward, if the building he has added on stands firm;15if it is burnt up, he will be the loser; and yet he himself will be saved, though only as men are saved by passing through fire.
vv. 10-15: In its immediate reference, this passage seems to imply that we cannot decide yet how much good the influence of any Christian teacher (e.g. Apollo) has done; it is only at the last day that such things will be made known to us. (Cf. 4.5 below.) But it has always been understood as having a wider application; each . . . of us is building on the foundation of Christian faith which has been laid in him, and the merits or demerits of his building will be made known only at his judgement. At the same time, we are to recognize that many whose actions in this world have had little value, will themselves escape condemnation, though only by passing through the fires of Purgatory.