Mark 4: “Then when they were by themselves, his close followers and the twelve asked about the parables, and he told them: ‘The secret of the kingdom of God has been given to you. But to those who do not know the secret, everything remains in parables, so that, seeing they may see, and not perceive; and hearing they may hear, and not understand lest haply they should turn again, and it should be forgiven them.'”
“So he taught them his message with many parables such as their minds could take in. He did not speak to them at all without using parables, although in private he explained everything to his disciples.” [Phillips translation]
Matthew 13: “The man who has ears to hear should use them”
“At this the disciples approached him and asked, ‘Why do you talk to them in parables?
“‘Because you have been given the chance to understand the secrets of the kingdom of Heaven,’ replied Jesus, ‘but they have not. For when a man has something, more is given to him till he has plenty. But if he has nothing even his nothing will be taken away from him. This is why I speak to them in these parables; because they go through life with their eyes open, but see nothing, and with their ears open, but understand nothing of what they hear.”‘ [Phillips translation]
1 Corinthians 2:6-15: “But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: which none of the princes of this world knew.
“We interpret what is spiritual in spiritual language. The unspiritual man rejects these truths of the Spirit of God; to him they are ‘sheer folly,’ he cannot understand them. And the reason is, that they must be read with the spiritual eye. The spiritual man, again, can read the meaning of everything; and yet no one can read what he is.”
Clement of Alexandria (150-220 C.E.)
“The Lord . . . allowed us to communicate of those divine Mysteries, and of that holy light, to those who are able to receive them. He did not certainly disclose to the many what did not belong to the many; but to the few to whom He knew that they belonged, who were capable of receiving and being moulded according to them. But secret things are entrusted to speech, not to writing, as is the case with God.”
“Many things, I well know, have escaped us, through length of time, that have dropped away unwritten.”
“Even now I fear, as it is said, ‘to cast the pearls before swine, lest they tread them underfoot, and turn and rend us.’ For it is difficult to exhibit the really pure and transparent words respecting the true Light to swinish and untrained hearers.”