The word "mystery," found in Mark 4:11,12 as in certain other places in Scripture, is not used in the classical sense of religious secrets or things incomprehensible, but of things of purely divine revelation-- matters foreshadowed in the ancient economy and then only partially understood, but now fully published under the Gospel (see I Cor. 2:6,10; Eph. 3:3,6,8,9). The mysteries of the kingdom meant those great Gospel truths which at that time none but the disciples could appreciate, and even they only in part, while to those without (whose hearts had not yet been opened to the Gospel) they were like tales and fables, subjects of entertainment rather than divine truths. Such persons saw but recognized not, and heard but understood not, for their spiritual sight and understanding were judicially sealed by sin. From obdurate rejection of the Gospel, and their obstinacy in preferring darkness to light, they had become morally incapable of acceptance and totally indifferent. (See prophecy of Is. 6:9,10, then read contrasting passage in Matt. 13:16.)
