Albert Barnes Commentary Psalms 98:3

"He hath remembered his lovingkindness and his faithfulness toward the house of Israel: All the ends of the earth have seen the salvation of our God." — Psalms 98:3 (ASV)

He hath remembered his mercy - Compare the notes on Luke 1:54-55 and Luke 1:72, where this passage in the Psalms was likely 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 forgotten it; 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 on that passage. The resemblance in the language is so strong as to make it probable that the psalm was composed after the time of Isaiah, and likely intended for use (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 suited to celebrate a higher deliverance—under the Messiah—of which the earlier deliverance was an emblem.