Charles Ellicott Commentary Isaiah 65:16

"so that he who blesseth himself in the earth shall bless himself in the God of truth; and he that sweareth in the earth shall swear by the God of truth; because the former troubles are forgotten, and because they are hid from mine eyes." — Isaiah 65:16 (ASV)

Shall bless himself in the God of truth ... —Literally, the God of the Amen. In Revelation 3:14, we have an echo of the Hebrew; in John 17:3, we have an equally distinct echo of the Septuagint rendering, τὸν θεὸν τὸν ἀληθινόν. These words seem to imply that the prophet had grasped the inner meaning of what was, for most people, only a liturgical formula.

Because the former troubles ... —The addition of this clause emphasizes the thought that it is the truth or faithfulness of God, who keeps His promise forever, that will lead people to use that new Name as a formula of benediction.