15 Therefore there has risen psaltery and harp in the dawn, and he confesses to the Lord; and says what? “I will confess to You among the peoples, O Lord, and will play to You among the nations: for magnified even unto the Heavens has been Your mercy, and even unto the clouds Your truth”. Heavens above clouds, and clouds below heavens: and nevertheless to this nearest heaven belong clouds. But sometimes clouds rest upon the mountains, even so far in the nearest air are they rolled.
But a Heaven above there is, the habitations of Angels, Thrones, Dominions, Principalities, Powers. This therefore may perchance seem to be what should have been said: “Unto the Heavens Your truth, and even unto the clouds Your mercy.” For in Heaven Angels praise God, seeing the very form of truth, without any darkness of vision, without any admixture of unreality: they see, love, praise, are not wearied. There is truth: but here in our own misery surely there is mercy. For to a miserable one must be rendered mercy.
For there is no need of mercy above, where is no miserable one. I have said this because that it seems as though it might have been more fittingly said, “Magnified even unto the Heavens has been Your truth, and even unto the clouds Your mercy.” For “clouds” we understand to be preachers of truth, men bearing that flesh in a manner dark, whence God both gleams in miracles, and thunders in precepts....Glory to our Lord, and to the Mercy of the Same, and to the Truth of the Same, because neither has He forsaken by mercy to make us blessed through His Grace, nor defrauded us of truth: because first Truth veiled in flesh came to us and healed through His flesh the interior eye of our heart, in order that hereafter face to face we may be able to see It. Giving therefore to Him thanks, let us say with the same Psalm the last verses, which sometime since too I have said, “Be Thou exalted above the Heavens, O God, and above all the earth Your glory”.
For this to Him the Prophet said so many years before; this now we see; this therefore let us also say.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)