How when and why did the earliest of american slaves become christians
or abondon whatever religion they brought with them from africa?

When:  According to <http://www.ipl.org/div/timeline/>, the English
government wanted slaves to be converted to Christianity as far back
as 1660.  In the 1680's, the English government told governors of the
colonies to convert slaves and Native Americans.

How: By law or at least urging of the monarchy, colonists were
encouraged to convert their slaves.  The Church of England sent
missionaries to America to convert them.  Also, slaveowners would
establish worship on their property, separate from the whites of the
household.  Slaves could be forced to attend church or meetings.  It
is reasonable to expect that many slaves turned to Christianity
without pressure or forced conversion.

Why:  Although some slaveowners believed that Christian slaves would
be more likely to try to escape (say, after learning the story of
Moses and the Israelites), most people believed that the Christian
message would teach slaves to accept their lot, work hard, and behave
as their owners wished.  On the one hand, spirituals sometimes held
encoded messages; but on the other hand, the lyrics mostly "provided
communal support for individual experiences"
<http://www.nhc.rtp.nc.us:8080/tserve/getback/gbafricanam.htm>

Africa had thousands of religions, mostly local. The processing, in
part caused deliberately by the transistion converted them. Many
tribes captured black folks from near by communities in Africa. Since
certain attitudes brought a better price, a few were kept a while, if
the tribe thought they might be able to make an attitude adjustment.
Tribes that were good at attitude adjustments typically bought the
slaves next. Still annother tribe typically moved the slaves to an
ocean port, where they met the first white men (typically Christains)
By then 3 or 4 radically different religions have been absorbed by
these mostly teen age slaves, including some far out variations on
Christianity. If the price of slaves was low in the USA, all or part
of a boat load was sold on an off USA shore island, for further
attitude adjustment, and making baby slaves. As often as not, few
pairs of people shared the same religion from their birth place, and
they would be separated if their keepers were aware of their having
the same religious roots. When a slave reached his final owner (always
weeks, occassionly years later) most had figured out life was more
certain if you pretended to be impressed with the master's religion,
and African ways were often punished; then totally forgoten in two or
three generations.   Neil