Clavis or The Key

It is written, The natural man receives not the things of the spirit, nor the Mystery of the kingdom of God, they are foolishness unto him, neither can he know them: therefore I admonish and exhort the Christian lover of Mysteries, if he will study these high writings, and read, search, and understand them, that he does not read them outwardly only, with sharp speculation and meditation; for in so doing, he shall remain in the outward imaginary ground only, and obtain no more than a counterfeit color of them.

From the Introduction:

2. For a man's own reason, without the light of them. God, cannot come into the ground [of them], it is impossible; let his wit be ever so high and subtle, it apprehends but as it were the shadow of it in a glass.

3. For Christ says, without me you can do nothing; and he is the Light of the World, and the Life of men.

4. Now if any one would search the divine ground, that is, the divine revelation, he must first consider with himself for what end he desires to know such things; whether he desires to practice that which he might obtain, and bestow it to the glory of God and the welfare of his neighbor; and whether he desires to die to earthliness, and to his own will, and to live in that which he seeks and desires, and to be one spirit with it.

5. If he have not a purpose, that if God should reveal himself and his Mysteries to him, he would be one spirit and have one will with him, and wholly resign and yield himself up to him, that God's spirit might do what he pleases with him and by him, and that God might be his knowledge, will, and deed, he is not yet fit for such knowledge and understanding.

6. For there are many that seek Mysteries and hidden knowledge, merely that they might be respected and highly esteemed by the world, and for their own gain and profit; but they attain not this ground, where the spirit searches all things, even the deep things of God: as it is written.