I guess the easiest illustration and application about slavery would be the fact that, because of the sin of Adam, and we being his offspring, we are born into the "slavery" of sin.  However, since Jesus was our ransom, he "purchased" us with his blood, so those who put faith in him (Heb 11:6), learn about him and his father (John 17:3) and are baptized in accordance with true knowledge, and not just on a whim (Eph 4:5), we become his slaves.