Steve didn't need to pull the shield up. Instead of going on the offensive, he went on the defensive. He didn't want to hurt this man. But there was only so long he could keep this up. Even with Natasha trying to keep him from using the knife, but Bucky seemed like he didn't want to be stopped. They needed to get the knife out of his hand. There were probably others, but damnit. They just needed to get him to listen. Believing was a separate issue.
Steve ducked the knife and working with his free hands to try and hit it out of the brainwashed man's hand. "I don't..." he ducked the next blow, "want to fight you. But we need you to listen. We aren't lying to you. We want to help you." The knife hit part of the armor on the suit, and Steve backed up, moving to shoulder the guy and knock him back or knock the knife loose. He took a quick second and pulled the mask down so his face was showing, hoping to job SOMETHING loose.