2 “Our God is a refuge and strength”. There are some refuges wherein is no strength, whereto when any flees, he is more weakened than strengthened. Thou fleest, for example, to some one greater in the world, that you may make yourself a powerful friend; this seems to you a refuge. Yet so great are this world's uncertainties, and so frequent grow the ruins of the powerful day by day, that when to such refuge you have come, you begin to fear more than ever therein....Our refuge is not such, but our refuge is strength. When there we have fled, we shall be firm.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)