There have been multiple fan theories regarding Steve Roger’s worthiness to wield Mjolnir after Age of Ultron first teased it, but one theory could potentially change how we look at The Infinity Saga as a whole.
One of the best scenes in the 2015 Avengers sequel is the one where the team takes turns trying to lift Thor’s hammer. When it comes to Captain America’s turn to try and lift, the Hammer almost budges, but ultimately Steve is unable to lift it. The two most common theories surrounding that scene suggest that Cap, being the humble all round nice guy that he is, for some reason decided to feign weakness, the other being what was implied but not said: that he wasn’t yet worthy enough.
Both suggestions don’t really make too much sense when you think about them, but a new theory shared by redditor, no_not_luke in the r/FanTheories subreddit suggests that Mjolnir consciously chose to move the way it did. In summary, the hammer planted the seeds for Steve to believe that he could be worthy in the future, but not until he lets go of the secret about the Winter Soldier and Tony’s parents. It’s all explained pretty in-depth by the redditor, who’s clearly thought pretty hard about all of this.
“because Mjolnir wobbled – not because Steve didn’t give it his all, but because he did use all his strength, and Mjolnir swayed to him – Steve realized that there was something inside him that could maybe one day lift the hammer. He may not have known what day the process was complete – or maybe he was wise enough to recognize it when it came – but the precedent of Mjolnir shifting under his touch was what gave him reason to try again when the universe depended on it.”
The theory also explains how Mjolnir could potentially experience the past, present, and future all in one, therefore knowing that the time will come in the future that Cap will need to wield Thor’s hammer and thus setting things in motion.
“Mjolnir does not perceive time the way humans do. We feel like we move from the present into a future that is created by the present, but – and this is the genuine science, in a nutshell – our perception is not the reality. […] But that’s time for humans. Something like a photon experiences everything in its “life”, from emittance to absorption, in no time at all; who’s to say a magic hammer with some sort of discerning non-human consciousness doesn’t have an idea of where it’s going?”
One hole in the theory is that the Mjolnir from Age of Ultron is destroyed by Hela in Thor: Ragnarok. The one that appears in Endgame comes from a different timeline, so it’s possible that Mjolnir is connected to tangential versions of itself.
If this is the case then it means that Mjolnir has been preparing Cap for Thanos for years and Cap was aware on some level of what he would have to do. It’s a pretty cool and well-thought-out theory that sounds a lot better than Cap just being “almost worthy”.