From a homily on the Beatitudes by St Gregory of Nyssa
Bodily health is a good thing, but what is truly blessed is not only to know how to keep one’s health but actually to be healthy. If someone praises health but then goes and eats food that makes him ill, what is the use to him, in his illness, of all his praise of health?
We need to look at the text we are considering in just the same way. It does not say that it is blessed to know something about the Lord God, but that it is blessed to have God within oneself. Blessed are the pure in heart, for they shall see God.
I do not think that this is simply intended to promise a direct vision of God if one purifies one’s soul. On the other hand, perhaps the magnificence of this saying is hinting at the same thing that is said more clearly to another audience: The kingdom of God is within you. That is, we are to understand that when we have purged our souls of every illusion and every disordered affection, we will see our own beauty as an image of the divine nature.
And it seems to me that the Word of God, in these few words, was saying something like this: In you there is a certain desire to contemplate what is truly good. But when you hear that God’s majesty is exalted high above the heavens, that his glory is beyond comprehension, that his beauty is beyond description, that his very nature can neither be perceived nor be understood, do not fall into despair or think you can never have the sight that you desire.
So if, by love and right living, you wash off the filth that has become stuck to your heart, the divine beauty will shine forth in you. Think of iron, which at one moment is dark and tarnished and the next, once the rust has been scraped off, shines and glistens brightly in the sun. It is the same with the inner core of man, which the Lord calls the heart. It has been in damp and foul places and is covered in patches of rust; but once the rust has been scraped off, it will recover itself and once more resemble its archetype. And so it will be good, since what resembles the good must be good itself.
Therefore, whoever looks at himself sees in himself what he desires. And whoever is pure in heart is blessed because, seeing his own purity, he sees the archetype reflected in the image. If you see the sun in a mirror then you are not looking directly at the sky, but still you are seeing the sun just as much as someone who looks directly at it. In the same way, the Lord is saying, although you do not have the strength to withstand the direct sight of the great and inaccessible light of God, if you look within yourselves once you have returned to the grace of the image that was placed in you from the beginning, you will find in yourselves all that you seek.
For to be God is to be pure, to be free from weakness and passion, to be separated from all evil. If these things are all true of you then God is within you. If your thought is kept pure from evil habits, free from passion and weakness, separated from all stain, you are blessed because your vision is sharp and clear. You are able to see what is invisible to those who have not been purified. The eyes of your soul have been cleansed of material filth and through the purity of your heart you have a clear sight of the vision of blessedness. What is that vision? It is purity, sanctity, simplicity, and other reflections of the brightness of the Divine nature. It is the sight of God.