keep in memory = hold fast. Greek. katecho. See 1Co_7:30.
what = with what word. Greek. logos.App-121. He refers to the substance of his preaching, based as it was on the facts of the Lord's death and resurrection, which last was challenged by some false teachers (1Co_15:12).
unless. See 1Co_14:5 (except).
have. Omit.
believed. App-150.
in vain = to no purpose. Greek. eike. See Rom_13:4.