At various times in the history of the Mysteries, which are far older than recorded history, those in the Mystery Schools were persecuted by powerful, dogmatic, and often corrupt interests. For this reason, the neophytes of the Mysteries were forced underground so that they could safely practice their craft and pursue their goals of self‑synthesis and the attainment of wisdom. Because they usually met in groups for mutual support, they needed secrecy to avoid discovery and persecution. 

Although Freemasonry is still often persecuted under totalitarian regimes, today, secrecy is not required for protection in free countries.
In medieval Europe, the operative stonemasons wanted to shield their craft from exploitation by incompetent workers. 

They also invested the skill and tools of their trade with private symbolic, moral, and esoteric significance. They adopted secrecy to reinforce the symbolism of their craft and assure that intruders did not learn their trade skills. Eventually, the operative or practicing Masons admitted to their Lodges some gentlemen interested in architecture and symbolism. 

The art and craft of Masonry thus came to have a speculative form, using symbolic tools and Masonic terminology as guidelines for the development of inner life. And the Masonic secrets became a way of talking about internal spiritual growth.

Today, the secrecy of Freemasonry is primarily symbolic. Many experiences of life cannot be put into words that will communicate them to others. That is true of relatively simple and trivial experiences, such as biting into a lemon. If you have never had that experience, no one can tell you what it is really like. If you have had it, you know firsthand the taste of a lemon, which no words can express.

The ineffableness of experience is even more accurate of life's great and moving experiences. No one can describe love so well as to put its essence into words. Love can be experienced; it cannot be told. Similarly, the experience of spiritual discovery, of awakening to the Reality within each of us, cannot be put into words. Love and spiritual discovery can be symbolized but not expressed in ordinary words. So too, the experiences that Freemasonry is designed to evoke are beyond language. 

Those experiences are symbolized by the objects, actions, and legends of Freemasonry, but they cannot be expressed adequately and directly in words. Masonic secrets are symbols of what cannot be expressed—the Masonic spiritual experience.

Masonic secrets are symbolic substitutes for the actual experiences that await the Freemason in the course of the inner initiations that are the existing content of Masonry.