Mark 4

To His believing disciples, the Savior taught -

"...Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables..." (New Testament | Mark 4:11)

Merriam-Webster dictionary defines mystery as -

"...a religious truth that one can know only by revelation and cannot fully understand..,"

I believe one can fully understand if the truth is received by revelation. This concept is the very core concept of the Church - to progressively receive revelation of that which be mysteries or truth. On this subject, Elder Bruce R. McConkie wrote -

"Though Jesus used parables to hide the full meaning of his teachings from the Jews, yet those rebellious hearers could have understood if they had sought the light, and their failure to believe and understand rose to condemn them. Jesus here tells them, in effect, that gospel light does not burst upon men in full noonday splendor, but that it arises in their hearts gradually, line upon line, precept upon precept, here a little and there a little. "He that receiveth light, and continueth in God, receiveth more light; and that light groweth brighter and brighter until the perfect day." (D. C. 50:24.) Eventually the faithful, having continued to grow in light and truth, shall have all things revealed to them and shall know all things. (D. C. 76:5-10; 93:26-28; 101:32-34; 121:26-29.) (DNTC 1:291.)