6 For many words are not needed, nor a long round of argument, but if we only consider his expression, we shall find the way that leads to it. He does not simply say, “Rejoice always;” but he adds the cause of the continual pleasure, saying, “Rejoice in the Lord always.” He who rejoices “in the Lord,” can not be deprived of the pleasure by any thing that may happen. For all other things in which we rejoice are mutable and changeable, and subject to variation. And not only does this grievous circumstance attend them, but moreover while they remain they do not afford us a pleasure sufficient to repel and veil the sadness that comes upon us from other quarters.
But the fear of God contains both these requisites. It is steadfast and immoveable, and sheds so much gladness that we can admit no sense of other evils. For the man who fears God as he ought, and trusts in Him, gathers from the very root of pleasure, and has possession of the whole fountain of cheerfulness. And as a spark falling upon a wide ocean quickly disappears, so whatever events happen to the man who fears God, these, falling as it were upon an immense ocean of joy, are quenched and destroyed!
This indeed is most to be wondered at, that while things which minister sadness are present, the man should remain joyful. For if there was nothing to produce grief, it would be no great matter to him that he was able continually to rejoice. But that at a time when he is urged to sadness by the pressure of many things, he is superior to all these, and is blithe in the midst of sorrow, this is truly a matter for astonishment! And as no one would have wondered that the three Children were not burnt, if they had remained far off from the furnace of Babylon!
(for the circumstance that astonished all was, that having been so long in such close contact with the fire, they left it more free from hurt than those who had not been in contact with it); so also we are able to say of the saints, that if no temptation had fastened itself upon them, we should not have wondered at their continual rejoicing. But the point worthy of admiration, and that which surpasses human nature, is this, that being encircled on all sides with innumerable waves, their condition is easier than that of those who enjoy an entire calm!
Source: Homilies on the Statues (New Advent)