This doxology (a short song of praise) ends Book Four (compare Psalms 41:13; 89:52).
from everlasting to everlasting: The reference to God’s eternal nature links to the opening of Book Four (Psalm 90:1–2).
Let all the people say, “Amen!”: The Hebrew word ’amen means “It is true.” The faithful accept the Lord’s will during exile and his promise to be their God.