Awake, awake, put on strength, O arm of the LORD; awake, as in the ancient days, in the generations of old. Are you not him who has cut Rahab, and wounded the sea monster?
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Eusebius of Caesarea
According to the Hebrew, it is not Jerusalem but the arm of the Lord that has to put on strength. This relates to what we spoke about above [v. 5], about the nations hoping in “my arm.” For we have said that the divinity of the Word is signified here, since he urges the people not to fear the rebuke of human beings or to be affected by their filth, and in the same way here it is right to see in the person of the people a prayer being sent to the “arm of the Lord” to rouse himself and overcome the enemies of his people. For we confess we are nothing unless you rise up and overcome on our behalf. - "Commentary on Isaiah 2.39"