There has to be a connection between 1 Corinthians 15 and the rapture. Without the change from mortal to immortal the rapture could not happen because....
Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; nor does corruption inherit incorruption
1 Corinthians 15:50
Paul explains the mystery of the change to glory that must take place before the rapture, a change in both the resurrected and those still alive, so there is a connection here.
My point has been that the mystery is not the rapture itself, but in what must take place before the rapture. In the context of 1 Corinthians 15, the mystery is described in the change to glory, which happens before the rapture takes place. So, there is a connection here, but sticking to my point, the rapture itself is not the mystery