1 Peter 1:13

Here the apostle begins his exhortations to those whose glorious state he had before described, thereby instructing us that Christianity is a doctrine according to godliness, designed to make us not only wiser, but better. I. He exhorts them to sobriety and holiness. 1. Wherefore gird up the loins of your mind, etc., Pe1 1:13. As if he had said, "Wherefore, since you are so honoured and distinguished, as above, Gird up the loins of your mind. You have a journey to go, a race to run, a warfare to accomplish, and a great work to do; as the traveller, the racer, the warrior, and the labourer, gather in, and gird up, their long and loose garments, that they may be more ready, prompt, and expeditious in their business, so do you by your minds, your inner man, and affections seated there: gird them, gather them in, let them not hang loose and neglected about you; restrain their extravagances, and let the loins or strength and vigour of your minds be exerted in your duty; disengage yourselves from all that would hinder you, and go on resolutely in your obedience. Be sober, be vigilant against all your spiritual dangers and enemies, and be temperate and modest in eating, drinking, apparel, recreation, business, and in the whole of your behaviour. Be sober-mined also in opinion, as well as in practice, and humble in your judgment of yourselves.

— Matthew Henry's Concise Commentary (public domain)

Wherefore--Seeing that the prophets ministered unto you in these high Gospel privileges which they did not themselves fully share in, though "searching" into them, and seeing that even angels "desire to look into" them, how earnest you ought to be and watchful in respect to them! gird up . . . loins--referring to Christ's own words, Luk 12:35; an image taken from the way in which the Israelites ate the passover with the loose outer robe girded up about the waist with a girdle, as ready for a journey. Workmen, pilgrims, runners, wrestlers, and warriors (all of whom are types of the Christians), so gird themselves up, both to shorten the garment so as not to impede motion, and to gird up the body itself so as to be braced for action. The believer is to have his mind (mental powers) collected and always ready for Christ's coming. "Gather in the strength of your spirit" [HENSLER]. Sobriety, that is, spiritual self-restraint, lest one be overcome by the allurements of the world and of sense, and patient hopeful waiting for Christ's revelation, are the true ways of "girding up the loins of the mind." to the end--rather, "perfectly," so that there may be nothing deficient in your hope, no casting away of your confidence. Still, there may be an allusion to the "end" mentioned in Pe1 1:9.

— Jamieson-Fausset-Brown Commentary (public domain)