Albert Barnes Commentary Isaiah 35:4

"Say to them that are of a fearful heart, Be strong, fear not: behold, your God will come [with] vengeance, [with] the recompense of God; he will come and save you." — Isaiah 35:4 (ASV)

Say to them - This is still an address to the ministers of religion, to make use of all the consolations which these truths and predictions provide to confirm and strengthen the people of God.

Of a fearful heart - Of a timid, fainthearted heart; those who tremble before their enemies. The Hebrew is, as in the Margin, ‘Of a hasty heart;’ that is, of those who are disposed to flee before their enemies (see the note at Isaiah 30:16).

Behold, your God will come with vengeance - That is, in the manner described in the previous chapter; and, generally, he will take vengeance on all the enemies of his people, and they will be punished. The language in this chapter is, in part, derived from the captivity at Babylon (Isaiah 35:10), and the general idea is that God would take vengeance on all their enemies and would bring them complete and final deliverance.

This does not mean that when the Messiah should come he would be disposed to take vengeance; nor do the words ‘your God’ here refer to the Messiah. It is meant that their God, Yahweh, would certainly come and destroy all their enemies, and thereby prepare the way for the coming of the Prince of peace.

The general promise is, that however many enemies might attack them, or however much they might fear them, yet Yahweh would be their protector, and would completely humble and defeat all their foes. The Hebrew will admit of a somewhat different translation, which I give in accordance with that proposed by Lowth. The sense is not materially varied.

Say you to the faint-hearted, Be strong; fear not; behold your God!
Vengeance will come; the retribution of God:
He himself will come, and will deliver you.