They [i.e. our physical afflictions] should, moreover, serve us for medicines to purge us from worldly affections, and retrench [i.e. 'remove'] what is superfluous in us, since they are to us the messengers of death, we ought to learn to have one foot raised to take our departure when it shall please God.Calvin is saying here that from our physical afflictions, whether it be disease, injury or the pains of old age, we ought to be learning to live as always ready to leave this present world to be with Jesus. We do not know when this will be, but God does. We ought to be prepared, when He sees fit, to leave all that we have in this world behind and enter into His glorious prescence. Our suffering is one of the ways that God prepares His people for this. They help to remove from us love for the world and its fleeting pleasures (see 1 John 2:15-17) that we might fix our affections on Jesus, in whom we have true and eternal riches.
This is to be our constant mindset as Christians, living as always ready to leave. We ought to be daily preparing for when we depart or Christ returns. This is to be true of us not only in the times of suffering (which makes this mindset easier), but also in the good times (when this mindset is easily forgotten). As Christians we are to be a people whose lives are shaped by always having one foot raised "to take our departure when it shall please God." Sam Storms, commenting on this letter of Calvin's says this: "Do we live every day with one foot lifted ever so deftly off the ground in constant alert and anxious expectation of the moment when we will depart this world and enter the splendor of heaven and the prescence of God himself?"