Writing in the most general terms, It seems to me that slavery is an ingrained human characteristic. This characteristic has been modified somewhat over the centuries by greater emphases on more humane values. E.g. the Declaration of Rights of Man, the influence of Wilberforce on making the slave trade illegal, to the 1948 UN Declaration of Human Rights, and right through to the 1976 law in India banning bonded labour and the UN sustainable development goals of 2015. These more humane values have come from people and organizations which are both religious and secular, and these continue right up to the present day.
However, on a personal level, the teachings of Jesus as exhibited in the NT, seem to suggest, at the very least, an ambiguity as to whether slavery was immoral or not. This might well be understandable when taking into account the historical context of his sayings, but if I am asked to judge his sayings as being the inspired word of God, then, because I think slavery is wrong, I have no alternative but to consider my personal morals on this particular point as being better than those that Jesus is reported to have conveyed/hinted at/ignored in the gospels. On this point, therefore, I am in general(but not total) agreement with Be Rational, who said:
I do not think slavery is moral, but your god does.
from Mess 63 'Who Created the Deity'.