21 These are great mysteries, brethren. How does the Spirit of God speak with us? How does it make us delights in this night? What is this, we ask you, brethren, whence are they sweeter, the darker they are? He mixes us our potion after His love, in certain wondrous ways. He makes His own sayings wondrous, so that while we were speaking what ye already knew, yet forasmuch as it was dug out of passages which seemed obscure, the knowledge itself seemed to be made new. Did ye not know, brethren, that the wicked are to be tolerated in the Church, and schisms not to be made? Did ye not already know, that within those nets which hold both good and bad fishes, we must abide even to the shore, nor must the nets be burst, because on the shore the good shall be separated into vessels, and the bad thrown away? You know this already; but these verses of this Psalm ye did not understand; that which you did not understand is explained; that which you knew has been renewed.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)