There are several Stranger Missions to complete in Chapter 4 of Red Dead Redemption 2's story, each with unique plots and characters.
For the Help A Brother Out Stranger Mission, you'll be assisting a monk, Brother Dorkins, in freeing slaves. You can find him near the entrance to the market in the East of Saint Denis, where he'll ask if you'd like to help the poor.
If you choose to donate to him, he'll tell you about a store he believes is holding slaves, which has been outlawed by this point in Red Dead Redemption 2's history. A marker will appear, so follow it to the store and look for the bookshelf.
You'll see marks on the floor and deduce that this is no ordinary bookcase, The pawnshop owner will loudly complain, but you can intimidate him. One of the books opens a secret door, so pull on the red book to open it. Or, use your brute strength and pull it back from the wall.
Once you're at the bottom of the stairs, you'll find two slaves. They don't speak any English, but you can free them both and lead them back to the monk. He'll offer you his collection dish as payment, which you can accept or refuse, depending on your moral compass.
Brothers and Sisters, One and All
Eventually, you'll come across Brother Dorkins again in the North of Saint Denis, on the porch of a church with some kids and Sister Calderon. One of the kids steals the sister's cross, and you need to track him down fast.
Turn left at the end of the road, then right past the man sitting on the corner. You'll then see him fighting with another man, so intervene and the kid will drop the cross and skip away. If you manage to find the boy within 19 seconds of running away, you'll get a gold medal for your troubles.
This is the same area where you'll find Mrs Downes, coincidently. She'll falsely claim to a policeman that you're bothering her, so escape the police and remain hidden. You might need to leave the city for a time, depending on where you choose to hide.
If you're quick enough, you can run back through the market and can hide in the courtyard of a house that might seem familiar from The Artist's Way Stranger Mission. Stay out of sight until you're no longer wanted then follow the waypoint back to see Sister Calderon.
Watch out for the policeman riding past the church - if he recognises you, you'll be wanted again. Hand the cross over to complete the mission. There's no monetary reward, but you do get a big chunk of Honor for your deeds.