I tend to think that Christ and the early church did oppose slavery, they just also opposed using force to end it.
I view the last supper (in part) as Jesus ceremonially freeing the disciples from slavery. Note that he specifically states "You are My Friends if you do whatever I command you. No longer do I call you slaves..." implying that they were indeed slaves formerly, but not anymore. He also called us to do the same from time to time, which would mean we are supposed to free our slaves too.
Slaves at the time were not permitted to dine with their masters (except during Saturnalia, which celebrated Saturn's rule during Golden Age when slavery did not exist and humanity was egalitarian and everyone prospered without having to labor). A master would not dine with his slave until he freed him. Often masters just asking the slaves they wished to free to dine with them and their friends instead of going though the more legalistic ceremonies associated with manumission. The other methods of freeing a slave would have been very inappropriate for Christ. In one, the court declared that the master was holding the slave in slavery unjustly, and demanded that he free the man after first having the master inflict corporal punishment (usually feigned) on the slave one last time. This would imply that God had no real authority over us, that he has treated us unjustly, there is a higher authority forcing him to free us against his will, and that he would vindictively punish us for seeking freedom from him. In the second method the slave would purchase his freedom using the money the master had allowed him to earn. This would imply that we can earn our salvation though good works and that there is some payment that God wants from us. By simply asking the slave to dine with him the master was declaring that he wished to free the slave of his own free will, not because the enslavement was unjust or because the slave had earned anything, but because the master loved the slave and would rather have a friend and an equal than human chattel. ("Equal" isn't quite right, as the social standing of the former slave would never be that high and because the freedman was still considered to have a moral although not a legal obligation to become a Client of the master and do favors for him.) (If the master kept treating the slave as a slave the slave could later take the master to court and demand his freedom using the testimony of the other guests as evidence. Actually, I believe that that was necessary in order to gain the full rights of citizenship like voting, although in the mean time they already had the same freedom and social standing they would have otherwise. This future appearance in court seems to be implied as what would happen in the Day of Judgment.)
I forget where, but I remember that one of the churches is strongly condemned for having the free and slaves seperate during Communion. It seems to me that they would do so to avoid having to free their Christian slaves as they had a spiritual obligation to do. In the book of Philemon Paul sent the slave Onesimus back to him master, but implored the master to accept him not as a slave but as a brother, so he was expecting him to be freed.
Christians were not supposed to defy or rebel against their masters, but Christian masters also had a duty not to oppress their slaves, never to hold a fellow Christian as a slave, and to share the gospel with everyone they could, which would certainly include his own slaves. It might have been nice if the Bible also said Christians should not hold non-Christians as slaves, but it is silent on that point. Slaves of Christians may have had a better life than they would have as poor freedmen though, plus they would know an easy way to become free if they want to.
In general Christianity largely agrees with Communism on many things, but it always insists that it be voluntary. It is strongly opposed to using force, so it is more Left Libertarian than Communist.
