The citta of someone attaining
the level of Arahant is completely purified at the moment of its
attainment. The question remains: When the body of an Arahant is
cremated, do the remaining bones become relics in each and every case
or not? From one Arahant to another, there is a considerable difference
in time between the moment when he reaches that attainment and the
moment when he finally passes away. The bones of living Arahants,
who maintain their body elements for a long period of time after their
attainment, are very likely to become relics after death. This is due
to the length of time involved. The citta of an Arahant maintains the
body elements by means of the various life-sustaining systems present
in the body, like the breath for example. At the same time, an Arahant
maintains throughout his daily activities an intrinsic level of samãdhi
that steadily works to cleanse his body elements until they also become
pure. This results in his bone remains becoming relics after he passes
away. But I’m not convinced that the bone remains of an Arahant, who
passes away shortly after his attainment, do become relics, since his
body elements were not subject to the same lengthy cleansing process
mentioned above.
An Arahant classified as dandhãbhiññã is one who attains enlightenment
slowly and gradually. He may well reach the Anãgãmï level, and
then be stuck there for a long time before he finally reaches the level
of Arahant. He must spend a lot of time investigating back and forth
between arahattamagga and arahattaphala before the citta develops sufficient
strength and skill to pass beyond. This process of investigating
arahattamagga for the sake of attaining arahattaphala is in fact an effective
means of cleansing the body elements. Having finally attained the level
of Arahant, his bones may well become relics after he passes away. On
the other hand, I am not at all sure that the bones of an Arahant, who
attains enlightenment quickly – that is, khippãbhiññã – and then passes
away shortly afterward, will necessarily become relics, since his purified
citta would have very little time to cleanse his body elements. As for the
ordinary citta of the average person, producing a transformation from
bone to relic is well beyond its capability.
...
Source: Biography of âcariya Mun-The Legacy, page 412-