The fact that slavery was accepted as a given throughout most of the history of Christianity
may be an important corrective to our understanding of the Christian life.
I have been reflecting on the recurrent scriptural theme that it is easier for 'the poor' to attain salvation than 'the rich'; and thinking about the symbolic 'pardon' which was given the peasantry in the medieval poem Piers Plowman by William Langland.
Here is the line of reasoning: If salvation is more possible for the poor, and slavery is not contradicted by Christianity, then salvation is certainly possible for slaves; however salvation may in fact be most possible for slaves.
It may be easier to be a good Christian as a slave (or some similar status, such as a serf or servant) than when free.
Contrariwise, it may be very difficult - or unusual - for a slave's master to lead a good Christian life, may be very unlikely that the slave owner will be able (or choose) to attain salvation.
Charles Williams makes much use of the term coinherence to describe the underlying unity of humankind - we live 'in' each other. Coinherence derives from the incarnation of Christ, which takes up humanity into God and means firstly that we 'dwell' in Christ and he in us, but also that all humans share this. So the unity, or relatedness of all humans is a religious fact.
The primary law is love of God, the second is love of neighbour; and Williams seems to describe the second law as derived from the first by the coinherence of Christ in all; in terms of a web of exchanges and substitutions between people - the dependence of people on each other, trading assistance both physical and spiritual, the one doing what the other cannot.
The web of exchange and substitution therefore ought to be willingly participated-in (as contrasted with the prideful desire to be independent of the web, to be autonomous). It is a secondary aspect of the primary dependence of everything on God as creator and sustainer, and of the necessity for the divine help known as Grace.
The main human task seems to be a humble and loving acceptance of this Grace.
This, if correct, clarifies the situation of a slave. The slave is, let us assume, a victim of circumstances; very explicitly dependent on his Master's will (or whim) and very unlikely to imagine that he is an autonomous agent, very unlikely to pursue this as an ideal.
By contrast with the slave (or poor man), the ultimate dependence of modern man (the rich man) is much more abstract, and furthermore obscured by all manner incoherent nonsense such as the concepts of 'rights' and 'freedom' and 'democracy' - which create an illusion of individual autonomy as the basis of reality.
The slave's Christian duty is (as for everyone) to do his best to pursue the Good under the circumstances in which he finds himself. Mostly he will be prevented from this, by his status. But that is not his fault. So long as he does his best, it is enough.
The slave Master, however, is an element in these circumstances - and has the ability to make it harder for the slave to live a Christian life, violating the second law. Doing his best for himself is not enough, if he damages the web of coinherence by constraining others.
Coinherence suggests that if the slave master prevents his slave from attending church, sharing the sacraments, learning scripture, meeting with other Christians, praying (or whatever constitutes proper Christian practice in that context) - then the sin is upon himself. And, of course, the sin comes from and is the attitude: the attitude that the slave's soul does not matter - the denial of shared humanity.
Modern man is, in innumerable ways, substantially in the position of the slave master; by many means impairing the ability of his neighbours to live the Christian life - consider the power of the journalist, the bureaucrat, the politician, the advertiser to force distractions on others; to distract others from the reality of life, to constrain the practice of the Good.
Indeed, there are no 'poor' in modern societies (almost none) and therefore everyone is in the position of the rich man of the scriptures; and modern society (outside the family) most resembles a web of interference and imposition (laws, regulations, rules, rights, taxes, subsidies) rather than a web of consenting mutual exchanges.
For Langland, salvation was more likely for a plowman (a serf) than a merchant (a free man).
In sum, if the institution of slavery is not incompatible with Christianity, then the status of a slave (or of something like a slave - a serf, indentured servant, a conscript, a soldier) may be more compatible with the Christian life than is modernity.
The life of a slave is not happier - obviously not - but it may be more compatible with salvation than modernity.