if there is no God then there certainly will be an afterlife.
-SBF
Why would there be reason to believe there would be an afterlife if there is no God?
. . .
Also, you do realize there are living people who are brain dead. A person can be alive and simultaneously a mental vegetable. I'd also point out that even within the narrow scope of our own personal experiences, we too are not guaranteed consciousness. There are several hours every night, between our states of REM while we sleep, where we entirely lack consciousness.