Rashi on TanakhPsalms 100

A song for a thanksgiving offering For thanksgiving, to recite it over thanksgiving offerings.

Serve the Lord with joy Now why so much? You should know the Lord is God, Who recompenses you with reward for your work, but the heathens need not worship with joy because their deities do not give them any reward.

He made us and we are His When we were not in the world.

and until generation after generation His faith will endure. Every expression of truth (אמת) and faith (אמונה) means the realization of a promise, that He makes true and assures His promise.