They were, with these dim but still purifying perceptions, unwilling to degrade the majesty of the First Great Cause by sharing his attributes with a Zeus and a Hera in Greece, a Jupiter and a Juno in Rome, an Osiris and an Isis in Egypt; and they did not believe that the thinking, feeling, reasoning soul, the guest and companion of the body, would, at the hour of that body’s dissolution, be consigned, with it, to total annihilation.
Hence, in the earliest ages after the era of the dispersion, there were some among the heathen who believed in the unity of God and the immortality of the soul. But these doctrines they durst not publicly teach. The minds of the people, groveling in superstition, and devoted, as St. Paul testifies of the Athenians, to the worship of unknown gods, were not prepared for the philosophic teachings of a pure theology. It was, indeed, an axiom unhesitatingly enunciated and frequently repeated by their writers, that “there are many truths with which it is useless for the people to be made acquainted, and many fables which it is not expedient that they should know to be false.”  Such is the language of Varro, as preserved by St. Augustine; and Strabo, another of their writers, exclaims, “It is not possible for a philosopher to conduct a multitude of women and ignorant people by a method of reasoning, and thus to invite them to piety, holiness, and faith; but the philosopher must also make use of superstition, and not omit the invention of fables and the performance of wonders.” 
While, therefore, in those early ages of the world, we find the masses groveling in the intellectual debasement of a polytheistic and idolatrous religion, with no support for the present, no hope for the future,–living without the knowledge of a supreme and superintending Providence, and dying without the expectation of a blissful immortality,–we shall at the same time find ample testimony that these consoling doctrines were secretly believed by the philosophers and their disciples.
But though believed, they were not publicly taught. They were heresies which it would have been impolitic and dangerous to have broached to the public ear; they were truths which might have led to contempt of the established system and to the overthrow of the popular superstition. Socrates, the Athenian sage, is an illustrious instance of the punishment that was meted out to the bold innovator who attempted to insult the gods and to poison the minds of youth with the heresies of a philosophic religion. “They permitted, therefore,” says a learned writer on this subject, “the multitude to remain plunged as they were in the depth of a gross and complicated idolatry; but for those philosophic few who could bear the light of truth without being confounded by the blaze, they removed the mysterious veil, and displayed to them the Deity in the radiant glory of his unity. From the vulgar eye, however, these doctrines were kept inviolably sacred, and wrapped in the veil of impenetrable mystery.”
The consequence of all this was, that no one was permitted to be invested with the knowledge of these sublime truths, until by a course of severe and arduous trials, by a long and painful initiation, and by a formal series of gradual preparations, he had proved himself worthy and capable of receiving the full light of wisdom. For this purpose, therefore, those peculiar religious institutions were organized which the ancients designated as the MYSTERIES, and which, from the resemblance of their organization, their objects, and their doctrines, have by Masonic writers been called the “Spurious Freemasonry of Antiquity.”
Warburton, in giving a definition of what these Mysteries were, says,
Each of the pagan gods had (besides the public and open) a secret worship paid unto him, to which none were admitted but those who had been selected by preparatory ceremonies, called initiation. This secret worship was termed the Mysteries.
I shall now endeavor briefly to trace the connection between these Mysteries and the institution of Freemasonry; and to do so, it will be necessary to enter upon some details of the constitution of those mystic assemblies.
Almost every country of the ancient world had its peculiar Mysteries, dedicated to the occult worship of some especial and favorite god, and to the inculcation of a secret doctrine, very different from that which was taught in the public ceremonial of devotion. Thus in Persia the Mysteries were dedicated to Mithras, or the Sun; in Egypt, to Isis and Osiris; in Greece, to Demeter; in Samothracia, to the gods Cabiri, the Mighty Ones; in Syria, to Dionysus; while in the more northern nations of Europe, such as Gaul and Britain, the initiations were dedicated to their peculiar deities, and were celebrated under the general name of the Druidical rites. But no matter where or how instituted, whether ostensibly in honor of the effeminate Adonis, the favorite of Venus, or of the implacable Odin, the Scandinavian god of war and carnage; whether dedicated to Demeter, the type of the earth, or to Mithras, the symbol of all that fructifies that earth,–the great object and design of the secret instruction were identical in all places, and the Mysteries constituted a school of religion in which the errors and absurdities of polytheism were revealed to the initiated. The candidate was taught that the multitudinous deities of the popular theology were but hidden symbols of the various attributes of the supreme god,–a spirit invisible and indivisible,–and that the soul, as an emanation from his essence, could “never see corruption,” but must, after the death of the body, be raised to an eternal life.
That this was the doctrine and the object of the Mysteries is evident from the concurrent testimony both of those ancient writers who flourished contemporaneously with the practice of them, and of those modern scholars who have devoted themselves to their investigation.
Thus Isocrates, speaking of them in his Panegyric, says,
Those who have been initiated in the Mysteries of Ceres entertain better hopes both as to the end of life and the whole of futurity.
Epictetus declares that everything in these Mysteries was instituted by the ancients for the instruction and amendment of life.
And Plato says that the design of initiation was to restore the soul to that state of perfection from which it had originally fallen.
Thomas Taylor, the celebrated Platonist, who possessed an unusual acquaintance with the character of these ancient rites, asserts that they
obscurely intimated, by mystic and splendid visions, the felicity of the soul, both here and hereafter, when purified from the defilements of a material nature, and constantly elevated to the realities of intellectual vision.
Creuzer, a distinguished German writer, who has examined the subject of the ancient Mysteries with great judgment and elaboration, gives a theory on their nature and design which is well worth consideration.
This theory is, that when there had been placed under the eyes of the initiated symbolical representations of the creation of the universe, and the origin of things, the migrations and purifications of the soul, the beginning and progress of civilization and agriculture, there was drawn from these symbols and these scenes in the Mysteries an instruction destined only for the more perfect, or the epopts, to whom were communicated the doctrines of the existence of a single and eternal God, and the destination of the universe and of man.
Creuzer here, however, refers rather to the general object of the instructions, than to the character of the rites and ceremonies by which they were impressed upon the mind; for in the Mysteries, as in Freemasonry, the Hierophant, whom we would now call the Master of the Lodge, often, as Lobeck observes, delivered a mystical lecture, or discourse, on some moral subject.
Faber, who, notwithstanding the predominance in his mind of a theory which referred every rite and symbol of the ancient world to the traditions of Noah, the ark, and the deluge, has given a generally correct view of the systems of ancient religion, describes the initiation into the Mysteries as a scenic representation of the mythic descent into Hades, or the grave, and the return from thence to the light of day.
In a few words, then, the object of instruction in all these Mysteries was the unity of God, and the intention of the ceremonies of initiation into them was, by a scenic representation of death, and subsequent restoration to life, to impress the great truths of the resurrection of the dead and the immortality of the soul.
I need scarcely here advert to the great similarity in design and conformation which existed between these ancient rites and the third or Master’s degree of Masonry. Like it they were all funereal in their character: they began in sorrow and lamentation, they ended in joy; there was an aphanism, or burial; a pastos, or grave; an euresis, or discovery of what had been lost; and a legend, or mythical relation,–all of which were entirely and profoundly symbolical in their character.
And hence, looking to this strange identity of design and form, between the initiations of the ancients and those of the modern Masons, writers have been disposed to designate these mysteries as the SPURIOUS FREEMASONRY OF ANTIQUITY.