Harris examines the Biblical evidence to show that the imagery of slavery is pervasive in Scripture, and that ultimately, we are slaves of Christ. Most of our English translations have softened it to servants, but the idea of slavery is very much there. Why slavery? It denotes complete ownership, command, protection and dependence (rather than autonomy). The slave must obey his Master's commands, and also seeks to please his Master.
I once attended a seminar taken by a leading Christian apologist to Muslims, who made the case that while in Islam the primary man-God relationship is that of slave, in Christianity we don't see ourselves as slaves of God. But this isn't the contrast that the NT makes. We are slaves - in fact, everyone is a slave to whatever masters him, as Peter wrote. So becoming a Christian is gaining a new master - Jesus is Lord! The difference with Islam is not that we do not see ourselves as God's slaves, but that we are slaves of the living and true God who revealed himself in Christ,
who, though he was in the form of God, did not count equality with God a thing to be grasped, but made himself nothing, taking the form of a slave, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross. [Phil.2]As Horatio Bonar's hymn asks, "It is the way the Master went: should not the servant tread it still?" We are not slaves of an arbritrary and capricious despot who only insists at length in writing that he is merciful: we are slaves of the one who came and was among us as one who serves, to give his life to purchase us for God. And that makes all the difference!
There are Scriptural limitations in the metaphor of slavery: Harris considers being friends of Christ, who are given revelation of his business, and adopted co-heirs of Christ. But we mustn't simply reject the image because it's not comfortable to us. As Christ's, we are his slaves, owned totally by Him, to obey Him fully and to live to please Him.
This may be despised by men, as if we might as well be dead, as live in slavery to a Master, following His will and living to please Him. But that's the point: I have been crucified with Christ: it is no longer I who live, but Christ who lives in me, and the life I now live in the flesh I live by trusting in the Son of God, who loved me and gave Himself for me. [Gal.2] What a joy and privilege!