Paul says why do they baptize for the dead referring to someone outside the church like by false teachers.
"Let us now return to the resurrection, to the defense of which against
heretics of all sorts we have given indeed sufficient attention in another work of ours. But we will not be wanting (in some defense of the doctrine) even here, in consideration of such
persons as are
ignorant of that little treatise. What, asks he, shall they do who are
baptized for the dead, if the dead rise not?
1 Corinthians 15:29 Now, never mind that practice, (whatever it may have been.) The
Februarian lustrations will perhaps answer him (quite as well), by
praying for the dead. Do not then suppose that the apostle here indicates some new
god as the author and advocate of this (
baptism for the dead. His only aim in alluding to it was) that he might all the more firmly insist upon the resurrection of the body, in proportion as they who were vainly
baptized for the dead resorted to the practice from their belief of
such a resurrection. We have the apostle in another passage defining but one
baptism.
Ephesians 4:5 To be
baptized for the dead therefore means, in fact, to be
baptized for the body; for, as we have shown, it is the
body which becomes
dead. What, then, shall they do who are
baptized for the body,
Ephesians 4:5 if the body rises not again? We stand, then, on firm ground (when we say) that the next question which the apostle has discussed equally relates to the body."
CHURCH FATHERS: Against Marcion, Book V (Tertullian)
What is the context of "In Alma 34:35, 36 we read: "For behold, if ye have procrastinated the day of your repentance even until death, behold ye have become subjected to the spirit of the devil, and he does seal you his. Therefore, the spirit of the Lord has withdrawn from you and hath no place in you; the power of the devil is over you, and this is the final state of the wicked.""( this is something I was from a google).
Mormonism’s Baptism for the Dead
Philip Schaff: NPNF1-12. Saint Chrysostom: Homilies on the Epistles of Paul to the Corinthians - Christian Classics Ethereal Library
Philip Schaff: ANF03. Latin Christianity: Its Founder, Tertullian - Christian Classics Ethereal Library
Philip Schaff: ANF03. Latin Christianity: Its Founder, Tertullian - Christian Classics Ethereal Library
Philip Schaff: NPNF2-05. Gregory of Nyssa: Dogmatic Treatises, Etc. - Christian Classics Ethereal Library