We have now discussed four parables. Next comes the parable of the treasure in the field, which begins in verse 44. But first we get an appendix, which follows the parable of the leaven.
34 All these things Jesus spoke to the multitudes in parables; and without a parable he did not speak to them,
35 that the word might be fulfilled which was spoken by the prophet, saying, I will open my mouth in parables, I will utter things hidden from the destruction.
only in parables
From Matthew 13 onwards, the Lord only speaks to the multitude in parables. By now it should be clear why He did this. Because of the unbelief of the people, the Lord hid His message in stories. The secrets of the Kingdom are hidden in the parables and the Lord makes them known only to believers.
prophecy
That the Lord spoke in parables was already foretold in the Hebrew Bible, ‘so that the word might be fulfilled, which was spoken by the prophet’. It is a quote or reference to Psalm 78, written by Asaph.
Psalm 78
2 I will open my mouth with a proverb, I will utter riddles of old.
This seems more like a free quotation than a literal quotation. Although there are always differences in the translation into other languages. But we may read the words of Jesus as a further explanation of the words in Psalm 78.
Earlier we saw that the Lord said that He spoke in parables, so that the multitudes seeing might not see, and hearing might not hear, nor understand, and that in this the prophecy of Isaiah was fulfilled. The actions and speech of Jesus were exactly as foretold in the Tenach