That expression may be confusing to us and maybe even sound funny. First-century Christians would have readily understood the metaphor. Orientals wore long, loose robes, which were not conducive to running or long journeys, so they would gather up the long robes with a girdle to prepare to run or travel and to provide greater ease and freedom of movement.
The book of First Peter is interesting in that it contains 30 Greek imperatives. That’s a lot of commands! The Christian life is a life of action and obedience to the will of God. Such action, therefore, if properly done, is to be preceded with thought, intent, purpose, desire, etc. In other words, the mind (i.e., heart) must be engaged! Before the first imperative was given, Peter told these Christians to remove all other hindrances and focus their minds on obeying God.
Are there things of this life that may hinder our minds from being truly “set…on things above” (Col. 3:1-2)? Let us “prepare [our] minds for action” (1 Pet. 1:13, NASB), and then, let’s act!