Luke 11

To the lawyers of His day, or those that interpreted what would consider, God given law, the Savior said -

"Woe unto you, lawyers! for ye have taken away the key of knowledge..." (New Testament | Luke 11:52)

The Prophet Joseph Smith added -

"...For you have taken away the key of knowledge, the fulness of the scriptures..." (JST)

These lawyers were supposedly the wise and prudent. But in prayer the Savior said -

"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." (New Testament | Luke 10:21)

Hiddent things are mysteries as defined by Merriam-Webster dictionary -

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

Without revelation, the wise and prudent of the world do not come to know religious truth. The key of knowledge then is revelation from the Holy Ghost. Revelation from the Holy Ghost only come by worthiness, faithfulness, and obedience to the commandments. For it is through -

:...this greater priesthood administereth the gospel and holdeth the key of the mysteries of the kingdom, even the key of the knowledge of God.
20 Therefore, in the ordinances thereof, the power of godliness is manifest." (Doctrine and Covenants | Section 84:19 - 20)