The Institutes, Book 4
Never Take Back What You Renounced
Beware, then, that you never take back any of the things you renounced and gave up. Do not, against the Lord’s command, return from the field of gospel work and be found to have put on again the coat you had stripped off. Do not sink back into the low and earthly lusts and desires of this world, and, defying Christ’s word, come down from the roof of perfection and dare to take up again any of the things you have renounced and given up. Beware that you remember nothing of your relatives or your former attachments, and that you are not called back to the cares and anxieties of this world, and so, as our Lord says, put your hand to the plow, look back, and be found unfit for the kingdom of heaven. Beware that, once you have begun to taste the knowledge of the Psalms and of this life, you do not little by little grow puffed up and think of reviving the pride that you have now, at your beginning, trampled underfoot in the ardor of faith and in the fullest humility. For then, as the apostle says, by building up again the things you had torn down, you would make yourself a backslider.
Rather, take care to continue to the very end in that state of nakedness which you professed in the sight of God and of his angels. And in this humility and patience too, with which you persevered for ten days before the doors and begged with many tears to be admitted into the monastery, you should not only continue but also grow and go forward. For it would be too bad if, when you ought to be carried on from the first rudiments and beginnings and go forward to perfection, you began to fall back from these to worse things. For not the one who begins these things, but the one who endures in them to the end, will be saved.