The Imitation of Christ, by Thomas a Kempis, tr. by William Benham [1886], at sacred-texts.com
"My Son, thou art not always able to continue in very fervent desire after virtues, nor to stand fast in the loftier region of contemplation; but thou must of necessity sometimes descend to lower things because of thine original corruption, and bear about the burden of corruptible life, though unwillingly and with weariness. So long as thou wearest a mortal body, thou shalt feel weariness and heaviness of heart. Therefore thou oughtest to groan often in the flesh because of the burden of the flesh, inasmuch as thou canst not give thyself to spiritual studies and divine contemplation unceasingly.
2. "At such a time it is expedient for thee to flee to humble and external works, and to renew thyself with good actions; to wait for My coming and heavenly visitation with sure confidence; to bear thy exile and drought of mind with patience, until thou be visited by Me again, and be freed from all anxieties. For I will cause thee to forget thy labours, and altogether to enjoy eternal peace. I will spread open before thee the pleasant pastures of the Scriptures, that with enlarged heart thou mayest begin to run in the way of My commandments. And thou shalt say, 'The sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us.'"(1)
(1) Romans viii. 18.