6:17 saith the Lord. See Isaiah 52:11. Note also the command to come out of “Babylon” Jeremiah 51:45; Revelation 18:4).

6:18 my sons and daughters. This promise is not quoted explicitly from any Old Testament passage, though the sense of it can often be detected. There are also many explicit New Testament passages that do assert that those who receive Christ become children of God (e.g., John 1:12; Romans 8:16). By direct inspiration, if nothing else, Paul could assure his readers that being separated unto God from the world, through receiving Christ, would indeed assure them that He would receive them as His sons and daughters.