It cannot be denied that Paul is here speaking of a vicarious baptism: one is baptised for the dead to ensure for them a share in the effect of baptism, and this must relate to a post-mortal life. It is also clear that Paul himself refers to this baptismal practice, and without distancing himself from it (This is the embarrassing perception which is the reason for some (comparatively few) interpreters making an imaginative attempt to ignore that this relates to a vicarious baptism).
Søren Agersnap, Baptism and the New
Life: A Study of Romans 6:1-14 (Langelandsgade, Denmark: Aarhus University
Press, 1999), 175-76.