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If the case is known, and the dying adult manifestly did not want any baptism, then when he is comatose or unconscious, it would not be advisable. HOWEVER, if there is any chance that he might have changed his mind, or, if a family member deeply desires to give the dying person every chance possible, or, if the dying person's disposition toward being baptized is not clearly known, or any other such circuмstance, there should not be any reason to forbid a CONDITIONAL Baptism.
The conditional baptism could be given with the intention that in the off chance that this dying person for whatever reason, at this time, wishes to be baptized, then, let this baptism be for that purpose, but if he does NOT want to be baptized, even given that we do not know this to be the case, then let this baptism be not effective.
And put the whole case into the hands of God, Who knows all things.
The form could be, "I conditionally baptize thee (insert name), in the Name of the Father, and of the Son, and of the Holy Ghost," making sure water is moving over the skin during mention of each of the three persons.
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