The doctrine of the resurrection is under attack (both then and now):
-- The enemies of the gospel deny it!
-- If there is no resurrection, one must be forced to accept the
following horrible conclusions:
1. The Easter story is a lie.
2. Preaching the gospel is useless.
3. Preachers are all liars.
a. Our trust in God is empty, worthless, and hopeless
b. We are still in our sin (15:17).
c. We are the most miserable of all creatures (15:19).
d. Those who live, suffer, and die for Christ are fools
5. Departed believers are forever dead, never to rise
6. In regard to this present life -- We should live it up,
for tomorrow we may die (15:32)!
-- Paul rebukes Christians who have been listening to the lies
of unbelievers about the resurrection.
What does "baptism for the dead" mean? Since personal faith is essential for salvation, it can't be that the practice saves or helps save (as Mormons believe). Paul doesn't commend or command it. He just recognizes that it's happening and uses it to show the absurdity of the practice if you don't believe in a literal resurrection! One possible understanding is that there was a period of time between belief and baptism. During this time there was usually some instruction in the faith. However, some new believers could have died before they were baptized. "Baptism for the dead" could be understood as a visible sign and celebration that those who died had legitimate faith and would therefore participate in the resurrection.