Charles Ellicott Commentary Isaiah 12

In that day you shall say... — The prophet becomes the psalmist of that new Exodus, and the hymn that follows is based upon the type of that in Exodus 15, though with less of local and historical colouring. He has been taught that confession must be blended with thanksgiving—that those only can rightly estimate the comfort which God gives who have first felt His wrath.

The fact that the prophet appears as a psalmist was a natural result of the training of the schools of the prophets, as described in 1 Samuel 19:20, possibly also of his familiarity with the Temple service as a priest or Levite. The group of psalms ascribed to the sons of Korah presents so many parallelisms to the writings of Isaiah, and so obviously belongs to the same period, that we may reasonably think of him as having been associated with that goodly company. (See Introduction.)

Behold, God is my salvation ... —The words also allow for the rendering, Behold the God of my salvation. In either construction, “salvation” is taken, as in the New Testament (John 4:22; 1 Peter 1:9–10), as meaning more than mere deliverance from danger, and including the highest spiritual blessings.

The Lord Jehovah ... —The Hebrew here and in Isaiah 26:4 presents the exceptional combination of the two Divine Names (Yah Yahveh). (See Psalm 68:4.) With this exception, the second clause of the verse is a verbal reproduction of Exodus 15:2.

Therefore with joy you shall draw water ... —Literally, And with joy. The words may be either part of the hymn, or addressed to those who are to join in it. The latter seems most in harmony with the context. In the later ritual of the Feast of Tabernacles, the priests went in solemn procession to the Pool of Siloam, filled a golden vase with water, carried it to the Temple, and poured it out on the western side of the altar of burnt offering, while the people chanted the great Hallel (Hymn of Praise) of Psalms 113-118 (See Note on John 7:37).

If we may assume that this represented the ritual of the monarchy, we may reasonably infer that the words of Isaiah pointed to it. The Talmud expressly connects the act with the symbolism of Isaiah’s words (Jerusalem Talmud, Sukkah 5:1), and the prophet’s reference to the waters of Shiloah in Isaiah 8:6 confirms the inference.

Declare his doings among the people. —Literally, among the peoples. The prophet quotes from the hymn which had been sung when the Ark was placed in Zion (1 Chronicles 16:8), and in part from Psalms 105:1.

For he hath done excellent things. — Here, again, the Hebrew indicates an echo from Exodus 15:1: He hath triumphed gloriously.