The Noblest of Men, and a Woman is one of the side quests available in Red Dead Redemption 2. During this mission, you have to look for certain gunslingers. The mission is put on hold after you find the first four gunslingers. However, you can still see it in your log. This page has the information on how to find these gunslingers and how to complete this quest in RDR2.
The Noblest of Men, and a Woman is given to you by Levin - you can meet this man in the smaller saloon in the eastern part of Valentine. This quest (marked with a white question mark on the map) is available in chapter 2 - after the gang breaks a camp in Horseshoe Overlook.
Arthur receives photos with whereabouts of four gunslingers. You can reach these people in any order. The three of the gunslingers have to be killed first and then photographed. One of them will agree to get photographed - this happens after a joined battle.
The quest doesn't end when you get the four photos. You also don't get any rewards. This isn't a bug - the quest will continue later.
Wait for the game to unlock the next stage of this quest - this happens in chapter 4. The quest marker for The Noblest of Men, and a Woman should appear in the same saloon in Valentine. Visit that place. Ask the bartender about Calloway and Levin.
The bartender tells you that the men are on a ship in Saint Denis. Go there and meet Levin. You receive a new objective - find Slim Grant.
The next mission objectives will start one after another. Arthur must find Slim Grant, free him, and take the man back to Levin. The last objective is to have a duel with Calloway.
You can do the same with other side quests that are still active in your log. Keep checking the map. The map legend informs you about new mission markers. Pay attention to new question markers and icons that inform you about receiving a new letter.