Peter uses an illustration from the time this was written. He makes a comparison with the clothing in Bible lands at that time. Men wore long robes which, for active work or running, were tucked up and tied around the waist with a belt, sometimes called a girdle. The expression means to tuck up your robe under your belt, and be ready for action. Peter tells us that holy living requires a mind-set that is focused on God and His Word.
Many believers live in a kind of complacent fog.
Their mind is like a flopping long robe, undisciplined and unfocused. That s the only way we ll be able to keep our minds free of the unnecessary things and focus on the glory of God. We are to be sober. This isn t about being under the influence of a substance that alters our behavior to an un-natural physical state. Be sober has to do with mental distraction by stimulants of the world.