Camy, I think historically, an attendee of Hannibal First Presbyterian
Church of 1847-53 would have been prone to consider such a lie a sin.
Slavery was a punishment designed by God for the descendants of Ham.  Slaves
were property.  God intended slaves to obey their masters.  Thus Huck,
within that context, would have offended God by protecting a slave.  The lie
would have aided in the theft of a slave -- a runaway slave steals himself
from his master, thus breaking one of the commandments.  Huck's clear moral
obligation was to instruct Jim to return to his master.
    I think Twain did not agree with these propositions as an adult --
certainly not when he wrote Huck.  However, the churches of his youth did.
Some eternal truths seem to change with time.
    Terrell Dempsey