The third of four installments of St Symeon the New Theologian's twenty-fourth discourse, "On Spiritual Knowledge" (continued from here):
This is natural, for the things that are sealed up and closed, unseen and unknown by all men, are opened up by the Holy Spirit alone. When they have thus been unveiled they become visible and knowable to us. How then will those who claim that they have never known at all the Holy Spirit's presence, radiance, illumination, and His coming to dwell in them have the power to know or perceive or think of them in any way? How shall they apprehend such mysteries, who have never at all experienced in themselves the recasting, renewal, transformation, reshaping, regeneration, that He brings about? Those who have not yet been baptized in the Holy Spirit (Mt 3:11), how can they know the change that comes over those who have been baptized in Him? Those who have not been "Born from above" (Jn 3:3), how shall they see the glory of those who have been "born from above" (as the Lord said), those who have been born of God (Jn 1:13) and have become the children of God (Jn 1:12)? Those who have refused to experience this, but by their negligence have missed this glory – for they have received the power to become such (cf. Jn 1:12) – tell me, what knowledge will enable them to understand or in any way imagine what the others have become?Taken from this edition.
God is Spirit (Jn 4:24), invisible, immortal, inaccessible, incomprehensible. Those who are born of Him He makes to be such as Himself, like the Father who has begotten them. They may be touched and seen in body only; in other respects they are known to God alone and know only Him; or, rather, they wish to be known to God alone (cf. Gal 4:9) and constantly strive to look to Him and are anxious to be seen by Him. To express it differently, just as the illiterate cannot read books like those who are literate, neither can those who have refused to go through the commandments of Christ by practicing them be granted the revelation of the Holy Spirit like those who have brooded over them and fulfilled them and shed their blood for them. The man who takes a sealed and closed book cannot see what is written in it nor can he understand its subject as long as the book remains sealed (Is 29:11), even though he may have learned all the wisdom of the world. Likewise even he who, as we have said, has learned all the divine Scripture by heart will never be able to know and perceive the mystical and divine glory and power hidden in them without going through all God's commandments and taking the Paraclete [Comforter] with him. [The Paraclete] will open to him the words like a book and mystically show him the glory they contain. Indeed, with the eternal life that causes them to spring forth, He will as well reveal the blessings of God hidden in them, blessings that are veiled and utterly invisible to the despisers and the negligent. This is to be expected, since they have nailed all their senses to the vanity of the world and are passionately attached to the pleasures of life and to physical beauty. But since the vision of their souls is dimmed, they are unable to see and contemplate the intellectual beauties of God's unutterable blessings.