For bodily endeavor, united to contrition of the spirit, will offer to God the sacrifice best pleasing to Him, and a worthy dwelling-place for holiness where the inmost recesses are pure and clean. But if, while we observe bodily fasting, we are entangled in these most deadly vices of the soul, affliction of the flesh will naught avail us while the more precious part of us is defiled, for we are offending in that very part of our nature wherein we become the dwelling-place of the Holy Spirit.
For it is not so much the corruptible flesh as the clean heart which is made the dwelling of God, and the Temple of the Holy Spirit. We must see, then, that while our outward man is fasting, the inner man must likewise keep himself from those evil meats of the soul; the inner man, I say, whom especially the blessed Apostle bids us present pure unto God, that he may be worthy to receive Christ in himself as his Guest.
~St John Cassian
Prayerful Life 