18 “They who trouble me cast me in the teeth.” Again that voice! “While they say daily unto me, Where is your God?”. And it is principally in the temptations of the Church they say this, “Where is your God?” How much was this cast in the teeth of the Martyrs! Those men so patient and courageous for the name of Christ, how often was it said to them, “Where is your God?” “Let Him deliver you, if He can.” For men saw their torments outwardly; they did not inwardly behold their crowns! “They who trouble me cast me in the teeth, while they say daily unto me, Where is your God?” And on this account, seeing “my soul is disquieted on account of myself,” what else should I say unto it than those words:
“Why art you cast down, O my soul; and why do you disquiet me?”. And, as it seems to answer, “Would you not have me disquiet you, placed as I am here in so great evils? Would you have me not disquiet you, panting as I am after what is good, thirsting and labouring as I am for it?” What should I say, but,
“Hope thou in God; for I will yet confess unto Him”. He states the very words of that confession; he repeats the grounds on which he fortifies his hope. “He is the health of my countenance, and my God.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)