The Enchiridion by St. Augustine
Chapter 63.--The Peace of God, Which Reigneth in Heaven, Passeth All Understanding.
This peace, as Scripture saith, |passeth all understanding,| and cannot be known by us until we have come into the full possession of it. For in what sense are heavenly things reconciled, except they be reconciled to us, viz. by coming into harmony with us? For in heaven there is unbroken peace, both between all the intelligent creatures that exist there, and between these and their Creator. And this peace, as is said, passeth all understanding; but this, of course, means our understanding, not that of those who always behold the face of their Father. We now, however great may be our human understanding, know but in part, and see through a glass darkly. But when we shall be equal unto the angels of God then we shall see face to face, as they do; and we shall have as great peace towards them as they have towards us, because we shall love them as much as we are loved by them. And so their peace shall be known to us: for our own peace shall be like to theirs, and as great as theirs, nor shall it then pass our understanding. But the peace of God, the peace which He cherisheth towards us, shall undoubtedly pass not our understanding only, but theirs as well. And this must be so: for every rational creature which is happy derives its happiness from Him; He does not derive His from it. And in this view it is better to interpret |all| in the passage, |The peace of God passeth all understanding,| as admitting of no exception even in favor of the understanding of the holy angels: the only exception that can be made is that of God Himself. For, of course, His peace does not pass His own understanding.