18 “Awake; why sleepest Thou, O Lord?”. Who is addressed, and who is the speaker? Would not he be more correctly said to sleep and slumber, who speaks such words as these? He replies to you, I know what I am saying: I know that “He that keeps Israel does not sleep:” but yet the Martyrs cry, “Awake; why sleepest Thou, O Lord?” O Lord Jesus, You were slain; Thou “slept” in Your Passion; to us You have now “awaked” from sleep. For “we” know that You have now “awaked” again.
To what purpose have You awaked and risen again? The Gentiles that persecute us, think You to be dead; do not believe You to have risen again. “Arise Thou” then to them also! “Why sleepest Thou,” though not to us, yet to them? For if they already believed You to have risen again, could they persecute us who believe in You? But why do they persecute? “Destroy, slay so and so, whoever have believed in You, such an one, who died an ill death!” As yet to them “Thou sleepest;” arise to them, that they may perceive that You have “awaked” again; and may be at rest.
Lastly, it has come to pass, while the Martyrs die, and say these things; while they sleep, and “awaken” Christ, truly dead in their sleepings, Christ has, in a certain sense, risen again in the Gentiles; i.e. it becomes believed, that He has risen again; so by degrees they themselves, becoming converted to Christ by believing, collected a numerous body: such as the persecutors dreaded; and the persecutions have come to an end. Why? Because Christ, who before was asleep to them, as not believing, has risen in the Gentiles. “Arise, and cast us not off for ever!”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)