# make known to you, brothers, the gospel

Possible meanings are 1) Paul wants to remind them by making the gospel known again, "help you remember, brothers, the gospel" or 2) he wants to make sure they understand the gospel, "make clear to you, brothers, the gospel."

# on which you stand

Paul is speaking of the Corinthians as if they were a house and the gospel as if it were the foundation on which the house was standing.