And when his disciples asked him what this parable meant, he said, "To you it has been given to know the secrets of the kingdom of God; but for others they are in parables, so that seeing they may not see, and hearing they may not understand. Now the parable is this: The seed is the word of God. The ones along the path are those who have heard; then the devil comes and takes away the word from their hearts, that they may not believe and be saved. And the ones on the rock are those who, when they hear the word, receive it with joy; but these have no root, they believe for a while and in time of temptation fall away. And as for what fell among the thorns, they are those who hear, but as they go on their way they are choked by the cares and riches and pleasures of life, and their fruit does not mature. And as for that in the good soil, they are those who, hearing the word, hold it fast in an honest and good heart, and bring forth fruit with patience. Luke 8:9–15 (RSV)
We can talk about the heart from a strictly biological understanding and should be concerned about its health. We can also talk about the heart as the mysterious center of spiritual life within each human being.
Jesus' parable of the sower is about the heart and the Word of God. A human heart has been created to live in response to the Word. In the world before Adam and Eve rebelled against their Creator, their hearts were perfectly tuned to the heart of the Father. When they surrendered to the temptation to be God, their hearts were estranged from the Father's heart.
Throughout the Scriptures, we see the Father appealing to the hearts of His people to return to Him. They do, for a time, then they turn away again. Still, the Father does not cease His efforts, sending His Son to take on human flesh, a human heart, to redeem our hearts.
Jesus does not pretend that every heart at all times is ready to hear the Word. At times our hearts are so hardened against God's Word that they are easy pickings for the devil. The Word has no time to take root, having no hope of growing there.
As Jesus teaches about the different states of the heart, we discover our hearts could fit into any one of the 'soils' in the parable. We could despair that our hearts may be trapped with no hope of change. We could despair if we were solely responsible for renewing our hearts.
It is important to remember that we may be powerless to change, but Jesus is not. Jesus has come to redeem humanity, all of humanity; this means our hearts as well. Our hearts can be renewed and made into good soil for the Word by our Lord through the Holy Spirit.