This follows on from the previous post, here.
What should Christians do when they see things in their society, country or culture taking a turn for the worse?
We should, of course, always pray. And since we are rational creatures, made in God's image, intended to live in a creation without death in it, we will naturally mourn, and desire that it were otherwise. We should seek to be salt and light, to show a better way, and to preserve the blessing of what remains.
What we should not, though, do is turn to fear and anger, as if God had promised that in the last days, no terrible times will come (2 Timothy 3:1ff), and that things will just get progressively better. Rather, we should understand that were that to happen, it would be very bad for the health of the Christian church. What our real situation is, both corporately and individually, is usually masked by prosperity, but revealed through trials. Moreover, earthly trials remind us of what the actual calling of God's people is. Even though the patriarchs were promised that their descendants would have an earthly inheritance, yet the patriarchs themselves understood that this was not their true hope or motivation in their actions. As Hebrews 11:13-16 says:
13 These all died in faith, not having received the promises, but having seen them afar off were assured of them, embraced them and confessed that they were strangers and pilgrims on the earth. 14 For those who say such things declare plainly that they seek a homeland. 15 And truly if they had called to mind that country from which they had come out, they would have had opportunity to return. 16 But now they desire a better, that is, a heavenly country. Therefore God is not ashamed to be called their God, for He has prepared a city for them.
Earthly trials are routinely sent by God as a spiritual blessing. He uses them as his means to sanctify his people, to mature them, and to prepare them for their true inheritance, in the presence of Christ and then at his return and the renewal of all things. Seeing things in this world rot, crumble or be torn down by plain stupidity or malice, performs a valuable spiritual function for us. We remember that we were not meant to seek our inheritance here, in this life anyway. Here, we are strangers and pilgrims. As God purifies and sanctifies us, so that we might be the pure bride of Christ, to be presented spotless in him on the last day, everything that is part of that process is ultimately a blessing, with a glorious end. It is towards that end that we should orient our hopes and desires. How foolish, then, how lacking in understanding, if the deep thought of our hearts when we enter into troubles is one of resentment and disappointment, as if God was doing something wrong? How worldly, if we can only judge things at the level of immediate outward appearance, and not understand (James 1) that we are to count it all joy, because God is working in us to lead us to a perfection beyond our imaginings?

I don’t agree that slavery is condoned in the Bible. To regulate is not to condone. Jesus explained to the Pharisees that when God gave regulations concerning divorce, this did not mean that it was compatible with his original intention, but was because of the hardness of men’s hearts (without any regulations, men would carry out de facto divorces that would leave their abandoned wives without any protection or ability to be legitimately received into another home - they would be destitute). Thus, Jesus teaches us to examine God’s original intention.
But to answer your question about teaching believers to dismantle slavery, I believe that this is implicit in Paul’s teaching both about slaves in general, and in the one specific named case that he handles. He does not explicitly command the immediate release of all slaves, in line with the general wisdom of God, that the gospel is intended to bless people, not curse them: if all Christians were simply to put their slaves out into the street immediately, this would be a greater evil than remaining as slaves in many cases. Rather, Paul teaches both masters and slaves that they are equal in the sight of God; if Christians, they are brothers in Christ, who descended to the lowest place for us all and gave up his rights for us, whose example we should follow. A Pharisaical mind will say “but there’s no direct command, so, I don’t have anything I’m required to do!”, but Scriptural law is case law, and we are expected to apply it to specific cases that haven’t been explicitly touched upon using the case law that we do have as a guideline, as per Exodus 21-24, and the Sermon on the Mount. The inevitable, unavoidable tendency of Paul’s teaching is to force slave-owners to face up to the fact that owning a slave, instead of paying a servant, is a violation of God’s law to love one’s neighbour (who is a divine image-bearer, not a possession) as oneself, and to lead one to voluntarily make that change.
Notice in Philemon that Paul states his apostolic authority to command Philemon, and that Philemon has an eternal debt to Paul for receiving the gospel… and yet Paul does not wish to use this authority; he wishes Philemon to release his slave voluntarily. The Pharisaical mind, as I say, refuses to admit the validity of any law that is not spelled out for him in words of one syllable in the imperative mood. The Bible’s approach is different; love is to work through wisdom, with discernment of the circumstances, and actions should generally be free.
It is, of course, implicit in this that retaining a particular slave was not necessarily (the circumstances of the enslavement might differ) in the same category as adultery, which must always be stopped immediately, no mitigating factors. I don’t claim that Paul demanded immediate total manumission (which as I say, would have caused significant harm). If you’re taking another man’s wife then you must always stop, immediately. Having a slave who perhaps lacked skills, opportunity or other resources to exist outside his present condition (usually as a consequence of having been enslaved) instead placed upon the slave-owner a moral responsibility to invest his own resources in order to move, at a wise pace, the slave into the position where he did have the resources to survive as a free person. So, it’s more akin to having fathered an illegitimate child: repentance doesn’t look like saying “this child should not be, so I shall cast it out”, but rather “this child is, because of my actions, so I am now responsible for it, and for mitigating the evil consequences of my actions in this child's life as much as possible”.
Another, perhaps clearer, analogy would be with polygamy. Polygamists were forbidden to be church leaders, because church leaders are required to not only teach the truth that all Christians should live up to, but to be actually be implementing it in their lives visibly as an example for both believers and outsiders. Nevertheless, polygamists in the church were not (as some missionaries have unfortunately taught, causing significant suffering) required to send all wives after the first away, disclaiming all further responsibility. Polygamy is an evil, and Paul’s teaching on marriage inevitably means the end of polygamy for disciples of Christ, but Paul’s method was not to demand immediate revolutionary actions that would cause innocent people to suffer. We find it hard to get our heads around that because of the abundance of our times, and because many of us have multiple potential safety nets if our circumstances change. But in the days of the Bible, things were otherwise.
So, I hold that Paul’s teaching placed a definite responsibility on the church to work definitely and energetically towards the end of slavery. The fact that the church has in fact being doing this since that time, with many notable successes, is not a case of “mission overreach”, but of obedience to God’s revealed will. The fact that in 19th century America professing Christians used their energy to oppose it and claimed that slavery in principle was legitimate and did not need to be dismantled, is a cause of shame. The analogy with polygamy is really a very good one here in my view. Apologists for polygamists really make the same arguments as slavery apologists (they observe its existence, and observe no command for its immediate abolition, and wrongly reason from that to divine approval), but neither can ever deal with the point that Jesus told us to look at God’s original intention for our race and to take that as the standard for all Christians living under the gospel.