This coming Sunday’s gospel ends: With many such parables he spoke the word to them as they were able to understand it. Without parables he did not speak to them, but to his own disciples he explained everything in private. (Mark 4:32-34) Mark concludes this collection of parables with a summary indicating that he has selected illustrations of Jesus’ teaching from a much larger cycle of tradition. It was Jesus’ method to teach the people through parables such as the one which Mark has presented. Through these parables Jesus is proclaiming the good news of the Kingdom; in other words, He was proclaiming “the word.” The term is an echo of the explanation given to the parable of the sower, where it occurs eight times. It is appropriate to the vocabulary of revelation and means clearly “the word of God,” or more concretely “the word of the Kingdom.”
The motive for Jesus’ use of the parables is expressed in terms of his accommodation to that stage of preparation which was present in the crowd; he spoke the word “as they were able to understand it.” This means that he adapted it to the level of understanding that he found in his listeners. Had Jesus spoken to the crowds in a direct manner they would have been forced to make a decision immediately. That decision could have expressed only unbelief and rejection. Jesus’ adoption of the indirect address of the parable was accordingly an expression both of grace and of judgment. It was an expression of grace which allowed time for reflection on his appeal to penetrate beneath his words to “the word.” It was an expression of judgment upon their lack of preparation to receive directly the word of the Kingdom of God. For that reason “he did not speak to them without a parable.”
With Jesus’ parables before the people the evangelist contrasts his private exposition of “everything” to his own disciples. “Everything” within this context means more than parabolic utterance; it refers to the mission of Jesus in which the mystery of the Kingdom was veiled. The summary, accordingly, points back to the contrast developed in Ch. 4:11–12 and exhibits the two aspects of the revelation of God in the mission of Jesus. There was veiling (or very partial disclosure) before the multitude and disclosure (but only partial understanding) to the disciples. This is the pattern illustrated in Ch. 4 and assumed throughout the Gospel of Mark. In the private instruction which Jesus gives to his disciples, the mystery of the Kingdom as present in his person is graciously unveiled. Only through revelation does the enigma become partially resolved; not until the consummation (to take the perspective of the parables) will it become resolved for all men.
The scholar Pheme Perkins  offers there reflections on this Markan passage.
“The seed parables point to the certain harvest that stems from next-to-invisible beginnings. The lack of human agency during the growth process does not mean that disciples should sit back and wait for God to bring the harvest. The proverbial sayings on how faith increases warn against such a conclusion. Instead, the image of a certain harvest from invisible beginnings promises that even though our testimony to the gospel appears insignificant or even fruitless, Christians should not be discouraged or give up. Christians should beware of giving in to the mania for statistics as evidence of success, which dominates modern life. When the harvest is ripe, it will be time for the reaper.”
“The image of a mustard bush as the kingdom of God set over against the alternative vision of the nations as great trees points to another feature of God’s rule. The kingdom does not replicate the kind of greatness that human nations attempt to build for themselves.”
“The passivity of human figures during the growth process challenges a common reading of these parables. They do not describe an evolutionary process by which Christians build the kingdom. The proverbial sayings warn Christians that faith cannot remain private. We must give away what we have received. This evangelical emphasis counters a common modern tendency to think of religion as a matter of private preference that is best worn lightly in the presence of others. These proverbs and parables suggest that God does not give the gift of faith (or secret of the kingdom) to individuals as their private possession. Rather, the gift provides light for others and shelter for the birds of the field.”
Pheme Perkins, The Gospel of Mark, vol. 8 of The New Interpreter’s Bible (Nashville, TN: Abington Press,1994)