Writings that failed to gain acceptance into the Old and New Testament canons were described in the writings of some early Christian scholars by the term “apocrypha.” The Greek word means “hidden things,” and when applied to books it described those works which religious authorities wished to be concealed from the reading public. The reason was that such books were thought to contain mysterious or secret lore, meaningful only to the initiate and therefore unsuitable for the ordinary reader. But the word “apocrypha” was also applied in a less complimentary sense to works that deserved to be concealed. Such works contained harmful doctrines or false teachings calculated to unsettle or pervert rather than edify those who read them. The suppression of undesirable writings was comparatively easy at time when only a few copies of any book were in circulation at a given time. Offensive writings would more likely have been burned by the authorities than “hidden” (compare Acts 19:19).
Hidden or esoteric teachings were not part of the Hebrew tradition, which based its spirituality on the first five books of the Hebrew canon. Insofar as mysterious doctrines came into Hebrew life, they did so from pagan sources and generally involved magical practices which were forbidden to Israel. Only when the concept of wisdom emerged in such writings as Proverbs, Ecclesiastes, Job and certain psalms did Jewish teachers such as Jesus Ben Sirach advise their hearers to search out the “hidden things” of divine wisdom (Ecclus. 14:20-21; 39:1-3, 7). Even so, the emphasis was upon knowing the mind and revealed will of God, not on the study of esoteric treatises of a kind popular among Hellenistic authors and readers.
R.K. Harrison, Old Testament and New Testament Apocrypha in The Origin of the Bible, Pages 83-84.
The “hidden things” of the apocrypha and wisdom literature were hidden in the sense that they were not suitable for the “ordinary reader.” They were suitable for “the initiate” - readers who were already steeped in the faith tradition and thus able to interpret the relative wisdom of the wise sayings through the prism of that faith. In other words, belief comes before wisdom.
This is consistent with Biblical admonitions such as “seek first the Kingdom of God… (Matt. 6:33)” and “the fear of the Lord is the beginning of knowledge (Prov. 1:7).”