Albert Barnes Commentary Isaiah 26:4

"Trust ye in Jehovah for ever; for in Jehovah, [even] Jehovah, is an everlasting rock." — Isaiah 26:4 (ASV)

Trust ye in the Lord for ever—The sense is, ‘Let your confidence in God on no occasion fail. Let no calamity, no adversity, no persecution, no poverty, nor any trial of any kind, prevent you from placing entire confidence in Him.’ This is spoken evidently in view of the fact stated in the previous verse: that the mind that is stayed on Him shall have perfect peace.

For in the Lord JEHOVAH—‘This is one of the four places where our translators have retained the original word Yahweh (Psalms 133:1–3:18; the notes at Isaiah 12:2).

The original is יהוה ביה b'yâhh y'hōvâh; the first word, יה y'âhh—(), being merely an abridged form of Yahweh. The same form occurs in Isaiah 12:2.

The union of these two forms seems designed to express, in the highest sense possible, the majesty, glory, and holiness of God, and to excite the highest possible reverence where language fails to convey the idea completely.

is everlasting strength—Hebrew as in the margin, ‘The rock of ages’—a more poetic and beautiful expression than in our translation.

The idea is that God is firm and unchangeable like an eternal rock, and that in Him we may find protection and defense for everlasting ages (see Deuteronomy 32:4, et al.; 1 Samuel 2:2; 2 Samuel 22:32, 47; 2 Samuel 23:3; Psalms 18:31; Psalms 19:14; Psalms 28:1; Psalms 42:9; Psalms 62:2, 6-7, etc., where God is called ‘a rock’).