Psalms 100, as its title tells, is a psalm of thanksgiving. “All ye lands” in verse 1 is properly read “all the earth,” or “all the land” (of Israel)
The ground or occasion for thanksgiving is “His mercy (or loving kindness) endureth forever, and His truth (or faithfulness) from generation to generation.”
Once His people, but having lost all title, Israel will again be His people, and the sheep of His pasture, because of immeasurable mercy. Thus His gates may he entered with thanksgiving and His courts with praise. Psalm 100 ends the series of psalms presenting the coming of Messiah, Jesus-Jehovah, to set up a rule of righteousness in the world, and to take the throne of Israel.