He hath remembered his mercy - Compare the notes at Luke 1:54-55 (note), Luke 1:72 (note), where this passage in the Psalms was not improbably referred to by Mary and Zacharias. The idea is, that God had called to mind his promise of mercy to his people; that he had not suffered it to pass out of his recollection; that he had kept his word.
And his truth - He has kept his promise; he has shown that he is a God of truth.
Toward the house of Israel - Toward his people.
All the ends of the earth have seen the salvation of our God - This appears to have been quoted from Isaiah 52:10. See the notes at that passage. The resemblance in the language is so strong as to make it probable that the psalm was composed after the times of Isaiah, and not improbably to be used (as remarked above) in the dedication of the temple after the captivity. The whole psalm would be appropriate to celebrate that deliverance; while, at the same time, like the language in Isaiah, it would be adapted to celebrate a higher deliverance - under the Messiah - of which that was an emblem.