Mat 13:10
And the disciples came, and said unto him, Why speakest thou unto them in parables?
Mat 13:11 He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given.
The word 'mystery', or 'mysteries', simply means a secret. Once Christ reveals the secret, it is no longer a mystery.
Isa 29:10
For the LORD hath poured out upon you the spirit of deep sleep, and hath closed your eyes: the prophets and your rulers, the seers hath he covered.
Luk 10:21 In that hour Jesus rejoiced in spirit, and said,
I thank thee, O Father, Lord of heaven and earth, that thou hast hid these things from the wise and prudent, and hast revealed them unto babes: even so, Father; for so it seemed good in thy sight.
So, Christ reveals His secrets to those who simply believe what He has written us in His Word, even if we do not yet understand it. For if we can come to Christ as an inocent child with trust, He has alot to work with, but if we come to Him already knowing everything [the wise and the prudent], He doesnt have much to work with, therefore, His secrets remain a mystery to many people.