1 Corinthians 15:29-34
John Lightfoot (d. 1664) understood τῶν νεκρῶν to refer to the dying martyrs, and interprets baptism metaphorically to refer to the baptism of suffering and martyrdom
John Edwards (1692) interprets this verse as referring to people who have themselves baptized as converts because they have witnessed the radiant confidence and courage of the martyrs
the dead refers to the decision of a person or persons to ask for, and to receive, baptism as a result of the desire to be united with their believing relatives who have died. This presupposes that they would share the radiant confidence that they would meet again in and through Christ at the resurrection of the dead
According to Meyer, this verse means that believers already baptized were rebaptized for the benefit of believers who had died unbaptized. This was done on the assumption that it would count for the unbaptized dead and thereby assure their resurrection along with the baptized, living believers. As Meyer put it, “This custom propagated and maintained itself afterwards only among heretical sects, in particular among the Corinthians (Epiphanius, Haer. 28:7) and among the Marcionites (Chrysostom; cf. moreover, generally Tertullian, de resurr. 48; Adv. Marc. v.10)” (The Epistles to the Corinthians, pp. 364, 365).