Matthew 12:43-45 Jesus was describing the attitude of the nation of Israel and the religious leaders in particular. Just cleaning up one's life without filling it with God leaves plenty of room for satan to enter. The book of Ezra records how the people rid themselves of idolatry, but failed to replace it with love for God and obedience to Him. Ridding our lives of sin in the first step. We must also take the second step: filling our lives with God's Word and the Holy Spirit. Unfilled and complacent people are easy targets for satan.
Matthew 13:2, 3 Jesus used many illustrations, or parables, when speaking to the crowds. A parable compares something familiar to something unfamiliar. It helps us understand spiritual truth by using everyday objects and relationships. Parables compel listeners to discover truth, while at the same time concealing the truth from those too lazy or too stubborn to see it. To those who are honestly searching, the truth becomes clear. We must be careful not to read too much into parables, forcing them to say what they don't mean. All parables have one meaning unless otherwise specified by Jesus.
Matthew 13:8 This parable should encourage spiritual "sowers" - those who teach, preach, and lead others. The farmer sowed good seed, but not all the seed sprouted, and even the plants that grew had varying yields. Don't be discouraged if you do not always see results as you faithfully teach the Word. Belief cannot be forced to follow a mathematical formula. But rather, it is a miracle of God's Holy Spirit as He uses your words to lead others to Him.
Matthew 13:9 Human ears hear many sounds, but there is a deeper kind of listening that results in spiritual understanding. If you honestly seek God's will, you will have spiritual hearing, and these parables will give you new perspectives.