Slavery in the British and American countries was an issue that found religious people arguing both sides. Biblical texts were used to argue both for and against; this usually means that the biblical texts are being misused. The modern social and cultural contexts are so radically different. Even in the Roman Empire, it was not the same. There were estimated to be over 60 million slaves in the Empire at one time; the Empire relied on slave labor for almost everything. Thus, an escaped slave (or a freedperson who assisted) was a serious threat to the empire. One only need read about the several slave uprisings (Spartacus being the most famous) to see this. Thus, early Christians seemed to tread lightly. Paul suggests that Philemon should free his slave, Onesimus, but he never comes out and states it outright. Some of the other letters in the New Testament use the model of a loyal slave as a model for how a Christian ought to serve the Master in heaven. The biblical texts seems to accept the practice of the time, without condoning it, and only obliquely condemning it. Not surprising.
What is surprising is how long it took western societies to ban it. (Or, perhaps not, when we consider how dependent societies were on slave labor. The financial results of its abolishment led to decades and even centuries of economic difficulties for the industries and people most dependent. It is a sad fact of humanity that sometimes economics trump being human.)
This Act abolished the slave trade
In the US, it was the 13th Amendment