Once you reach Champion rank in the Argent Tournament, this daily quest will open up for all Tournament participants no matter your faction. It can be acquired from Squire Artie within the Argent Pavilion and is among the most straightforward of daily quests. Artie is in charge of the makeshift chapel's collection plate, and is asking you to make a donation. You give him 10 gold, you get 100 reputation with the Argent Crusade as a reward.
Like the Sunwell Plateau's A Charitable Donation, this quest's only purpose is to let you get some easy added rep. If you don't feel like knocking out that last Heroic to finish your trip to Exalted, you can just do this quest. Do you have a ton of gold but don't have much time to run heroics for reputation at all? Do this quest on a daily basis, and you'll eventually get rewards.
You don't have to do this quest, and not doing it has no negative impact on your Tournament progress. When you reach Exalted with the Argent Crusade, you no longer receive any reward for completing this quest. I'm sure a number of you will think to yourselves, 'Did this quest really need a guide?' Considering how many people stare dumbfounded at this Squire on my home server trying to figure out his purpose, I say yes. It did need one.