The Imitation of Christ, by Thomas a Kempis, tr. by William Benham , at sacred-texts.com
"My Son, thou canst not possess perfect liberty unless thou altogether deny thyself. All they are enslaved who are possessors of riches, they who love themselves, the selfish, the curious, the restless; those who ever seek after soft things, and not after the things of Jesus Christ; those who continually plan and devise that which will not stand. For whatsoever cometh not of God shall perish. Hold fast the short and complete saying, 'Renounce all things, and thou shalt find all things; give up thy lust, and thou shalt find rest.' Dwell upon this in thy mind, and when thou art full of it, thou shalt understand all things."
2. O Lord, this is not the work of a day, nor children's play; verily in this short saying is enclosed all the perfection of the religious.
3. "My son, thou oughtest not to be turned aside, nor immediately cast down, because thou hast heard the way of the perfect. Rather oughtest thou to be provoked to higher aims, and at the least to long after the desire thereof. Oh that it were so with thee, and that thou hadst come to this, that thou wert not a lover of thine own self, but wert ready always to My nod, and to his whom I have placed over thee as thy father. Then shouldest thou please Me exceedingly, and all thy life should go on in joy and peace. Thou hast still many things to renounce, which if thou resign not utterly to Me, thou shalt not gain what thou seekest. I counsel thee to buy of Me gold tried in the fire, that thou mayest be rich,(1) that is heavenly wisdom, which despiseth all base things. Put away from thee earthly wisdom, and all pleasure, whether common to men, or thine own.
4. "I tell thee that thou must buy vile things with those which are costly and great in the esteem of men. For wonderfully vile and small, and almost given up to forgetfulness, doth true heavenly wisdom appear, which thinketh not high things of itself, nor seeketh to be magnified upon the earth; many honour it with their lips, but in heart are far from it; it is indeed the precious pearl, which is hidden from many."
(1) Revelation iii. 18.