SECOND LONDON BAPTIST CONFESSION OF FAITH 1.7-1.8
Some things in Scripture are clearer than others, and some people understand the teachings more clearly than others. However, the things that must be known, believed, and obeyed for salvation are so clearly set forth and explained in one part of Scripture or another that both the educated and uneducated may achieve a sufficient understanding of them by properly using ordinary measures.
The most likely interpretation is that some Corinthians had undergone baptism to bear witness to the faith of fellow believers who had died without experiencing that rite themselves. Paul’s reference to the practice here is neither a recommendation nor a condemnation. He simply uses it as evidence from the lives of the Corinthians themselves to bolster his larger argument, begun in 15:12, that resurrection from the dead is a present reality in Christ and a future reality for them. Whatever they may have proclaimed, the Corinthians’ actions demonstrated that they had hope for a bodily resurrection.