Isaiah 55:1-3 contains an appeal, continuing the theme that there is a spiritual food that nourishes the inner man and fills one's life in a way and with abundance that all of a person's material things cannot. That God is speaking about His Word is seen in the word "listen," which is directly connected to the phrase "eat what is good." This food is, of course, spiritual, and its source is God. Interestingly, He says to come and buy, but not with money. This food cannot be purchased with material wealth. All the money in the world cannot purchase it, but it still must be bought. Recall that the foolish virgins in Matthew 25 are advised to go out and buy oil from those who sell in preparation for the coming of the Bridegroom.
The "food" in Isaiah 55 and the "oil" in Matthew 25 can be bought only by means of the dedication and commitment of one's life in submission to Christ. By being a living sacrifice in prayer, study, meditation, and obedience, one becomes energized by the food of God's Word. In addition, one can "purchase" it only from those appointed by God to "sell" it. It can only be bought from those already converted and provided by God with the gifts to teach it to others. In most cases, this is the ministry of the true church.
Jeremiah 3:15 provides us with clear Old Testament evidence that the principle of feeding the mind with the correct instruction leads to good spiritual health: "And I will give you shepherds according to My heart, who will feed you with knowledge and understanding." God clearly states that a mind fed with the right things can produce wisdom, holiness, and happiness. In other words, He promises that those who hear Him will be fed the elements of an abundant life through shepherds who exhibit godly character. God's Word, if it is believed and practiced, produces a unique perspective of life and a balance that cannot be found through any other means. Nothing that man has produced through philosophy or religion can even come close. These elements of human society have played major roles in producing restless, anxious, violent cultures.
We must choose to secure the best diet for the mind to utilize and assimilate into one's moral and spiritual character, as well as other expressions of personality. The world produces an almost overwhelming amount of spiritual junk food and outright spiritual garbage, and it is within easy reach of any mind anywhere no matter where one lives.