There have been various theories throughout the years as to what this refers.

• Martin Luther believed it was an ordinary baptism of a living person, but that it occurred over the tomb of the dead.

• John Calvin saw this as a normal baptsim of someone when they were close to death.

• Another interpretation is that this is a metaphor and someone being baptized has a view towards death

• This also could mean vicarious baptism for the dead--people being baptized on behalf of the dead that had not been baptized.

Ultimately, we don't know what was going on, since this is the only reference to the practice.

As a side note, some Christians practice baptism for the dead during modertn times. This baptism for the dead is a vicarious baptism for the sake of one who is dead but was never baptized.

Source

There have been various theories throughout the years as to what this refers.

• Martin Luther believed it was an ordinary baptism of a living person, but that it occurred over the tomb of the dead.

• John Calvin saw this as a normal baptism of someone when they were close to death.

• Another interpretation is that this is a metaphor and someone being baptized has a view towards death

• This also could mean vicarious baptism for the dead—people being baptized on behalf of the dead that had not been baptized.

Ultimately, we don't know what was going on, since this is the only reference to the practice.

As a side note, some Christians practice baptism for the dead during modern times. This baptism for the dead is a vicarious baptism for the sake of one who is dead but was never baptized.

Source