Christianity in general is not "against" slavery. Individual Christian sects can take a positive, neutral or negative stance on it.

There is no prohibition of slavery in the Bible - the Old Testament outright accepts it, the New Testament tolerates it, even kinda encourages it:

https://www.biblegateway.com/passage/?search=Ephesians+6%3A5-9&version=NIV

"Ephesians 6:5-9 New International Version (NIV) 5 Slaves, obey your earthly masters with respect and fear, and with sincerity of heart, just as you would obey Christ. 6 Obey them not only to win their favor when their eye is on you, but as slaves of Christ, doing the will of God from your heart. 7 Serve wholeheartedly, as if you were serving the Lord, not people, 8 because you know that the Lord will reward each one for whatever good they do, whether they are slave or free.

9 And masters, treat your slaves in the same way. Do not threaten them, since you know that he who is both their Master and yours is in heaven, and there is no favoritism with him."

https://www.biblegateway.com/passage/?search=Colossians+3%3A22-24&version=NIV

"Colossians 3:22-24 New International Version (NIV) 22 Slaves, obey your earthly masters in everything; and do it, not only when their eye is on you and to curry their favor, but with sincerity of heart and reverence for the Lord. 23 Whatever you do, work at it with all your heart, as working for the Lord, not for human masters, 24 since you know that you will receive an inheritance from the Lord as a reward. It is the Lord Christ you are serving."

https://www.biblegateway.com/passage/?search=1+Timothy+6%3A1-2&version=NIV

"1 Timothy 6:1-2 New International Version (NIV) 6 All who are under the yoke of slavery should consider their masters worthy of full respect, so that God’s name and our teaching may not be slandered. 2 Those who have believing masters should not show them disrespect just because they are fellow believers. Instead, they should serve them even better because their masters are dear to them as fellow believers and are devoted to the welfare[a] of their slaves."

https://www.biblegateway.com/passage/?search=1+Peter+2%3A18&version=NIV

"1 Peter 2:18 New International Version (NIV) 18 Slaves, in reverent fear of God submit yourselves to your masters, not only to those who are good and considerate, but also to those who are harsh."

https://www.biblegateway.com/passage/?search=Titus+2%3A9-10&version=NIV

"Titus 2:9-10 New International Version (NIV) 9 Teach slaves to be subject to their masters in everything, to try to please them, not to talk back to them, 10 and not to steal from them, but to show that they can be fully trusted, so that in every way they will make the teaching about God our Savior attractive."

The Antebellum Southern Christian theologians were writing whole learned treatises on the topic, to wit:

https://quod.lib.umich.edu/cgi/t/text/text-idx?c=moa;idno=ABT6096

From this book you can learn how slavery was justified theologically.