ANSWER:

Baptism by proxy (substitution) cannot be reconciled with Christianity which is why 1 Corinthians 15:29 is not endorsing such. We cannot interpret a difficult passage to do violence to clear and understandable ones. Mormon dogma does just that. Jesus taught once one is in the Hadean realm that their state is fixed and cannot be changed (read Lk. 16:19-31). Secondly, the Bible teaches that we must bear our own personal responsibility which makes the whole notion of proxy baptism collapse. Note:

John 7:17, "If anyone wants to do His will, he shall know concerning the doctrine, whether it is from God or whether I speak on My own authority." Note: "if anyone wants to."

Romans 14:12, "So then each of us shall give account of himself to God."

2 Cor. 5:10, "For we must all appear before the judgment seat of Christ, that each one may receive the things done in the body, according to what he has done, whether good or bad."

Note "according to what he has done." Not, according to what someone else has done to/for him.

Acts 26:27-29, "'King Agrippa, do you believe the prophets? I know that you do believe.' Then Agrippa said to Paul, 'You almost persuade me to become a Christian.' And Paul said, 'I would to God that not only you, but also all who hear me today, might become both almost and altogether such as I am, except for these chains.'"

Perhaps Paul should not have worried so much about whether Agrippa and "all who hear me today" should become persuaded? Why not just wait till they die and then get baptized for them? Right? This is the kind of nonsense that Mormonism sows. If anything, it helps people "put off" baptism and thus be lost.

Also consider:

1. The context of 1 Corinthians 15 is about the resurrection, not baptism (v. 12). Baptism is presented here as an argument for the resurrection.

2. Since baptism by proxy is not taught in the NT (this is the only place anyone can go to have a hint of proof for it); since we are taught thoroughly that eternity is based on our personal decision and not the decision of another, then we must understand the meaning of this baptism the baptism that is for the remission of sin and place blame on the "translation" of the phrase as being problematic. Therefore the most probable understanding of 1 Cor. 15:29 is being baptized could bring hostility upon one's self, why be baptized in view of the dead (that is death) if the dead do not rise. Baptism identifies one with Christ and could bring jeopardy, "and why do we stand in jeopardy every hour?" (v. 30) if the dead do not rise?

3. The best translation I have read of this passage comes from "God's Word to the Nations Version" (GWV) which translates it as:

"However, people are baptized because the dead will come back to life. What will they do? If the dead can’t come back to life, why do people get baptized as if they can come back to life?"