Claude Mariottini, who isn’t affiliated with the ESV, delves into the textual background of Psalm 100:3.
The King James Version translates it:
Know ye that the LORD he is God: it is he that hath made us, and not we ourselves; we are his people, and the sheep of his pasture.
The ESV translates it:
Know that the LORD, he is God! It is he who made us, and we are his; we are his people, and the sheep of his pasture.
 Or and not we ourselves
Many other modern translations use phrasing similar to the ESV’s. Read Claude’s post to find out why.