The answer is: Philemon

A little book, only 25 verses, found just before Hebrews.

The letter to Philemon is a fascinating peek into early Christian life and spirituality, especially when it comes to concepts like redemption and forgiveness. Onesimus, the slave in question, had run away from Philemon, who was likely a wealthy Christian convert living in Colossae. When Onesimus encountered Paul – possibly during his time in prison – he underwent a significant personal transformation.

Now, interesting trivia: this short letter is like a private conversation that slipped into the public realm. In fact, it’s one of the only works in the New Testament focusing specifically on the theme of slavery. Paul's appeal to Philemon is profound; he asks him not simply to forgive Onesimus as a runaway slave but to receive him as a brother in Christ. This infectious spirit of unity, disregarding social classes, would have been revolutionary at the time.

Also noteworthy is textual depth—when Paul mentions they will "have him back forever" (Philemon 1:15), it's not just referring to physical return, but hints at the higher relationship they can all share as equals under the beliefs of their faith.

Moreover, while discussing this area of society, it’s essential to remember how the reality of societal norms at that time can differ vastly from current views. Paul doesn’t outright condemn the institution of slavery but gently redirects the focus on personhood, grace, and treating others with literal heartfelt dignity. This letter may seem brief, but it leaves a long-lasting impact, emphasizing unity over division in communities.