For ever.--The Hebrew ('olam) has properly neither the abstract idea of negation of time, nor the concrete (Christian) idea of eternity, but implies indefiniteness, and looks either backwards or forwards.
With my mouth--i.e., aloud, or loudly.
with my mouth will I make known thy faithfulness to all generations; God is faithful to himself, to all the perfections of his nature, to his truth, holiness, and justice, he cannot deny himself; he is so to his Son, and to all engagements with him, and promises to him; to all his counsels, purposes, and decrees; all which are faithfulness and truth, or faithfully and truly performed; and to his covenant and promises made to his people in Christ, in whom they are all yea and amen: and that this glorious perfection of God might be made known to the saints in all successive generations, and be taken notice of by them, the psalmist spoke and sung this psalm with his mouth, and penned it with his hand; in which there is more mention made of the faithfulness of God than perhaps in any other passage of Scripture besides; see Psalm 89:2.
(k) Works, vol. i. p. 699, 700. (l) "bonitates", Tigurine version; "benignitates", Junius & Tremellius; "beneficia", Piscator; "gratias", Cocceius.