Paul does not write in this chapter about whether slavery is good or bad. Paul teaches about working to please God whether as a slave or as a master. What Paul teaches here about slavery would have been surprising. In his time, masters were not expected to treat their slaves with respect and not threaten them.
This extended metaphor describes how Christians can protect themselves when spiritually attacked. (See: [[rc://en/tw/dict/bible/kt/spirit]] and [[rc://en/ta/man/translate/figs-metaphor]])