9] Awake, awake, put on strength, O arm of the Lord; awake, as in days of old, in the generations of old. Are you not the one that struck Rahav and slew the sea monster (Tanin)? 10] Are you not the one that dried the sea (Yam), the waters of the great deep (Tehom); who made the depths of the sea a way for the redeemed to pass over?
ט עוּרִ֨י עוּרִ֤י לִבְשִׁי־עֹז֙ זְר֣וֹעַ ה' ע֚וּרִי כִּ֣ימֵי קֶ֔דֶם דּוֹר֖וֹת עוֹלָמִ֑ים הֲל֥וֹא אַתְּ־הִ֛יא הַמַּחְצֶ֥בֶת רַ֖הַב מְחוֹלֶ֥לֶת תַּנִּֽין: י הֲל֤וֹא אַתְּ־הִיא֙ הַמַּֽחֲרֶ֣בֶת יָ֔ם מֵ֖י תְּה֣וֹם רַבָּ֑ה הַשָּׂ֨מָה֙ מַֽעֲמַקֵּי־יָ֔ם דֶּ֖רֶךְ לַֽעֲבֹ֥ר גְּאוּלִֽים:
Isaiah 51 is a deep well of mythical ideas - the prophet describes the people calling on the Arm of God from exile, saying that just as the arm of God slew the sea monster in the mythic past, it should redeem us today from our troubles.
Note that each part of these two verses may be referring to a different event. Verse 9 describes the victory over the chaos monster in ancient times (known as the chaoskampf , that we already saw in Psalm 74). The beginning of verse 10 (drying up the sea and the great deep) seems to refer to the Flood narrative, a prime example of victory over the chaos waters. The end of verse 10, referring to the passage of the redeemed, refers to the splitting of the Red Sea in Exodus 15. Then verse 11, that describes how the ransomed of God will return home, is a prediction about the future redemption. The speakers argue that since God has defeated the monster many times, both in the primordial past, at the flood and at the exodus, God can do it again (David Gunn, Journal of Biblical Literature, '75).
We've already seen the terms Tanin, Yam and Tehom in mythological contexts, though Isaiah 51 ties all of them together very neatly. The new term for the chaos monster here is Rahav but we won't dwell on it too much here. Instead, I want to focus on the tone of these verses - why call on the Arm of God? Why not God God's self?
