Can Unclean Food Defile a Man?

To those who claim that the Scriptures in Matthew 15:11 condemn the Church's law of abstinence and fasting, Dom Gueranger in his "Liturgical Year" addresses the following words:

[God] tells them that there is no creature which is intrinsically, and of its own nature, unclean; and that a man’s conscience cannot be defiled by the mere fact of his eating certain kinds of food. Evil thoughts, and evil deeds, these, says our Saviour, are the things that defile a man. Some heretics have interpreted these words as being an implicit condemnation of the exterior practices ordained by the Church, and more especially of abstinence. To such reasoners and teachers we may justly apply what our Saviour said to the pharisees: They are blind and leaders of the blind. From this, that the sins into which a man falls by his use of material things are sins only on account of the malice of the will, which is spiritual, it does not follow that therefore man may, without any sin, make use of material things, when God or His Church forbids their use. 

God forbade our first parents, under pain of death, to eat the fruit of a certain tree; they ate it, and sin was the result of their eating. Was the fruit unclean of its own nature? No; it was a creature of God as well as the other fruits of Eden; but our first parents sinned by eating it, because their doing so was an act of disobedience. Again, when God gave His Law on Mount Sinai, He forbade the Hebrews to eat the flesh of certain animals; if they ate it, they were guilty of sin, not because this sort of food was intrinsically evil or cursed, but because they that partook of it disobeyed the Lord. 

The commandments of the Church regarding fasting and abstinence are of a similar nature. It is that we may secure to ourselves the blessing of Christian penance— in other words, it is for our spiritual interest—that the Church bids us abstain and fast at certain times. If we violate her law, it is not the food we take that defiles us, but the resisting a sacred power, which our Saviour, in yesterday's Gospel, told us we are to obey under the heavy penalty which He expressed in those words: He that will not hear the Church, shall be counted as a heathen and publican.