The Russo Brothers had recently claimed that Captain America could always lift Mjolnir in Avengers: Age of Ultron. Thor’s hammer Mjolnir is the most powerful weapons in the MCU and thanks to the enchantment by Odin, only a person who is worthy of the hammer can lift it. Thor had been deemed to be unworthy in his first stand-alone film before he had earned his worthiness.
In Avengers: Age of Ultron, there was a scene dedicated to every Avengers trying to prove their worthiness and lift the hammer. War Machine and Iron Man could not lift the hammer with their combined strength and Hawkeye had also tried and failed. Black Widow had refused to even try. Captain America’s attempt had him slightly move the hammer, and Thor was a little shocked. This had been a tease that he is worthy and may pick up the hammer sometime in the future. Vision had then picked it up later on in the movie.
The conversation about this scene had sparked up after the movie, Avengers: Endgame had released on theatres and had shown Captain America lifting the Mjolnir, and he had complete control over its powers during his battle with Thanos.
During a recent interview with Happy Sad Confused Podcast, the Russo brothers had been asked why Cap is worthy to lift the hammer now:
“In our heads, he was able to wield it. He didn’t know that until that moment in Ultron when he tried to pick it up. But Cap’s sense of character and humility and, out of deference to Thor’s ego, Cap in that moment realizing he can move the hammer, decides not to.”
In Endgame, after Cap had wielded the Mjolnir, Thor said, “I knew it” and this is proof that Thor also knew that Cap was worthy all along.