Here’s what a few commentaries I have say about the verse:
**29. ***For what good will they do …’ *The meaning is very doubtful. Perhaps some persons at Corinth had desired to become Christians but had been overtaken by death before they could be baptised and some Christian friends had been allowed to undergo a (purely symbolical) baptism for them to show that the dead were counted as Christians. If so, the practice was soon dropped for fear of misunderstanding. Here again and in 30–32 Paul seems to be dealing with a teaching that dead Christians had perished.
Rees, W. (1953). 1 and 2 Corinthians. In B. Orchard & E. F. Sutcliffe (Eds.), A Catholic Commentary on Holy Scripture (p. 1097). Toronto;New York;Edinburgh: Thomas Nelson.
Ver. 29. Who are baptized for the dead. He still brings other proofs of the resurrection. This is a hard place, and the words are differently expounded. 1. Several late interpreters understand a metaphorical baptism, and that to be baptized for the dead, is to undertake self-denials, mortifications, and works of penance, in hopes of a happy resurrection; and this exposition agrees with what follows, of being exposed to dangers every hour, of dying daily, &c. But if this had been the apostle’s meaning, he would rather have said, Who baptize themselves. Besides, this exposition is not so much as mentioned in any of the ancient interpreters. 2. Some think that S. Paul tells the Corinthians that they ought not to question the resurrection of the dead, who had a custom among them, if any one died without baptism, to baptize another that was living for him; and this they did, fancying that such a baptism would be profitable to the dead person, in order to a happy resurrection. Tertullian mentions this custom in one or two places, and also S. Chrys. on this place. But it does not seem probable that S. Paul would bring any argument of the resurrection from a custom which he himself could not approve, nor was ever approved in the Church. 3. S. Chrys. and the Greek interpreters, who generally follow him, expound these words, who are baptized for the dead, as if it were the same as to say, who receive baptism with hopes that they themselves, and all the dead, will rise again; and therefore make a profession, when they are baptized, that they believe the resurrection. So that S. Paul here brings this proof among others, that they who have been made Christians, and continue Christians, cannot call in question the resurrection, which they professed to believe in their creed at their baptism, the creed being always repeated before they were baptized. 4. Others, by being baptized for the dead, understand those who begged and called for baptism when they were in danger of death, and would by no means go out of this world without being baptized, hoping thereby to have a happy resurrection of their bodies; so that to be baptized for the dead is the same as on the account of the state of the dead, which they were entering into. See S. Epiphan. hær. viii. p. 114. Edit Petavii. Wi. Some think the apostle here alludes to a ceremony then in use: but others, more probably, to the prayers and penitential labours performed by the primitive Christians for the souls of the faithful departed: or to the baptism of afflictions and sufferings undergone for sinners spiritually dead.
Haydock, G. L. (1859). Haydock’s Catholic Bible Commentary (1 Co 15:29). New York: Edward Dunigan and Brother.
thanks for answering my question. 