From the subject of prayer the Master turned to that of fasting, and emphasized the important truth that to be of avail fasting must be a matter between the man and his God, not between man and his kind. It was a common thing in the Master's day to see men parading the fact of their abstinence as an advertisement of their assumed piety.[541] That they might appear haggard and faint, this class of hypocrites disfigured their faces, went with unkempt hair, gazed about with sad countenances. Of these also the Lord said, "Verily I say unto you, They have their reward." Believers were admonished to fast secretly, with no outward display, and to fast unto God, who could see in secret and would heed their sacrifice and prayer. TREASURES OF EARTH AND OF HEAVEN.[542] The transitory character of worldly wealth was next contrasted with the enduring riches of eternity. Many there were and many there are whose principal effort in life has been that of amassing treasures of earth, the mere possession of which entails responsibility, care, and disturbing anxiety. Some kinds of wealth are endangered by the ravages of moths, such as silks and velvets, satins and furs; some are destroyed by corrosion and rust--silver and copper and steel; while these and others are not infrequently made the booty of thieves. Infinitely more precious are the treasures of a life well spent, the wealth of good deeds, the account of which is kept in heaven, where the riches of righteous achievement are safe from moth, rust, and robbers. Then followed the trenchant lesson: _"For where your treasure is, there will your heart be also."_ Spiritual light is shown to be greater than any product of physical illuminants. What does the brightest light avail the man who is blind? It is the bodily eye that discerns the light of the candle, the lamp, or the sun; and the spiritual eye sees by spiritual light; if then man's spiritual eye be single, that is, pure and undimmed by sin, he is filled with the light that shall show him the way to God; whereas if his soul's eye be evil, he will be as one full of darkness. Solemn caution is expressed in the summary, "If therefore the light that is in thee be darkness, how great is that darkness!" Those whom the Master was addressing had received of the light of God; the degree of belief they had already professed was proof of that. Should they turn from the great emprise on which they had embarked, the light would be lost, and the succeeding darkness would be denser than that from which they had been relieved.[543] There was to be no indecision among the disciples. No one of them could serve two masters; if he professed so to do he would be an untrue servant to the one or the other. Then followed another profound generalization: _"Ye cannot serve God and mammon."_[544]