Notice the order: stillness comes before knowing, not after. We tend to assume it works the other way, that once we've sufficiently reasoned our way to trusting God, the stillness will follow as a reward. This verse reverses that. The stillness isn't the result of having your questions answered; it's the posture that makes the knowing possible in the first place. The verse doesn't stop at personal reassurance either. It widens out to “I will be exalted among the nations, in the earth.” The command to be still isn't only about your private anxiety. It's set against the backdrop of God's authority over everything, including the things that feel the most unmanageable to you specifically. Your stillness doesn't change God's exaltation. But maybe your stillness is what lets you actually see it.