|An early Christian baptistery in the shape of a cross.|
“Else what will those being baptized for the dead do? If in fact the dead are not raised, why indeed are they being baptized for them?” (1 Cor. 15:29)
Q: I noticed that this verse says something about baptizing for the dead. What does that mean? –Tammie H.
A: The short answer is that nobody knows for sure. The long answer is that they were probably being baptized for believers who had died before they were baptized.*
* This was the guess, about 300 years later, of Epiphanius (4th cent.; Panarion 28:6.3-5). Chrysostom (also 4th cent.) attributes a similar practice to followers of the heretic Marcion (Homily 40 on First Corinthians). But Tertullian (2nd/3rd cent.) expresses ignorance of what exact practice this verse referred to (Against Marcion, 5.10).
We know that in the early centuries of Christianity, many intentionally put off baptism because they were afraid of the consequences of post-baptismal sin. This was because of a common teaching that it was not possible to be forgiven of certain types of sin after baptism, so people put off baptism until just before death. (The most famous example of this intentional delay is the Emperor Constantine, who was baptized just before he died.) It's possible that this concern was already circulating in some places in Paul's day.
In any case, the people doing this type of baptism probably thought it would be a good idea since the gospel message was that you must believe and be baptized in order to be saved (Mark 16:16, John 3:5, 1 Pet. 3:21, etc.). For the friends of a believer who died without baptism, a baptism in their name probably seemed like a good idea, just to be safe.
However, it's important to note that while Paul doesn't directly approve or disapprove of this practice, he attributes it to others, and not to his own churches (“they,” not “you” or “we”). As a result, the practice soon faded from sight. It was replaced initially with the understanding that a believer that was martyred before baptism was baptized in his own blood—which in times of persecution became the most common reason a believer would die without being baptized.
Soon after that followed the practice of infant baptism for the children of believers, also just to be sure, in a time when infant mortality was high. There was also the introduction of the penitential system, which provided a means of dealing with post-baptismal sin.
(For more on this topic, see the index category Baptism.)