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 as distinct an echo of the LXX. rendering, τὸν θεὸν τὸν�. The words seem to imply that the prophet had entered into the inner meaning of what was to most men only a liturgical formula.
Because the former troubles... — The addition of the clause emphasises the thought that it is the truth or faithfulness of God, who keepeth His promise for over, that will lead men to use that new Name as a formula of benediction.