Why Jesus used parables

“The disciples came to Him and asked, ‘Why do You speak to the people in parables?’ He replied, ‘The knowledge of the secrets of the kingdom of heaven has been given to you, but not to them. Whoever has will be given more, and he will have an abundance. Whoever does not have, even what he has will be taken from him.’” Matthew 13:10-12
In Matthew 13 we see a sudden shift in Jesus’ ministry. Before this He spoke freely to the people, such as at the Sermon on the Mount (which was directed at the disciples, but others listened in also). After this, when He spoke in public, it was primarily using parables. The reason for this was in Matthew 12, the Pharisees showed that they had completely hardened their hearts against Him, accusing Him of healing people by the power of Satan (Matt. 12:22-24).
Not only was it prophesied that Jesus would teach in parables (Ps. 78:2), but they also served a double purpose, which Jesus explains in today’s verse. On the one hand, for those who had come to know God and had received spiritual understanding from Him, parables serve as an illustrative tool, teaching spiritual truths using physical analogies. On the other hand, for those who are spiritually blind, the parables served to keep that truth from them. The reason for this is not malicious, rather it is gracious: they would be judged less severely, because they were ignorant of the truths being taught. If Jesus had spoken plainly, and they had rejected it, their judgement would be more severe.
The Bible is still the same today. To someone who is not born again, it is merely a collection of religious writings. But to someone who is born again, who has the Holy Spirit indwelling them, it is powerful and alive. “This is what we speak, not in words taught us by human wisdom but in words taught by the Spirit, expressing spiritual truths in spiritual words. The man without the Spirit does not accept the things that come from the Spirit of God, for they are foolishness to him, and he cannot understand them, because they are spiritually discerned” (1 Cor. 2:12-13).