343 Industries have confirmed that simply playing regular MP matches won't help you progress through the Battle Pass. Instead, you'll need to complete specific challenges.
Basically, just playing Halo Infinite multiplayer matches won't get you far in Battle Pass. You will have to complete specific challenges to earn XP and these challenges will have requirements like playing and winning matches. This will help you progress through the Battle Pass.
Here's what Halo Community Manager John Junyszek said regarding the Battle Pass progression:
"Playing and winning matches will be challenges, which will help players progress through the Battle Pass. Even though this means no per-match XP at launch, you're still always progressing through challenges and therefore the battle pass."
He added that Halo Infinite Tech Preview had an issue that caused people to run out of challenges but this will be sorted out before launch. He does not promise an infinite amount of challenges but there will be plenty of them so you probably won't ever run out of daily tasks to complete.
"Our current plans for launch, while not 'infinite' mean it's extremely difficult to run out daily challenges. I won't say 'impossible," cause there are some grinders out there, but I'd be impressed."
Some players probably won't be happy that playing regular matches won't reward you with XP, especially those who don't have time to play the game regularly. However, let's wait for the launch to see how exactly will this work and if there's any reason for concern.
Halo Infinite is coming December 8, 2021, to PC, Xbox One and Xbox Series X|S.