Now lettest - Now thou “dost” let or permit. This word is in the indicative mood, and signifies that God was permitting him to die in peace, by having relieved his anxieties, allayed his fears, fulfilled the promises, and having by the appearing of the Messiah, removed every reason why he should live any longer, and every wish to live.
Depart - Die.
According to thy word - Thy promise made by revelation. God never disappoints. To many it might have appeared improbable, when such a promise was made to an old man, that it should be fulfilled. But God fulfils all his word, keeps all his promises, and never disappoints those who trust in him.