The foundation metaphor: Jesus Christ is the foundation; building on it = post-salvation ministry to believers

The foundation Paul laid was the gospel itself — Christ and salvation by faith. What you 'build on it' is everything that happens in ministry after people are saved: discipleship, teaching, counseling, exhortation. This passage is primarily addressed to ministers/leaders building on the faith of existing believers, not about the individual believer's personal purification after death.