In the Epistle to the Assembly at Ephesus, the reference is to Exodus: to God's love in choosing them out of Egypt, and them making them a nation. See Hos. xi.1: "When Israel was a child, then I loved him, and called my son out of Egypt." In Jer. ii.2 we see this "first love," which Jehovah calls "the love of thine espousals." And in Ezek. xvi. we have a full description of the workings and outcome of this "first love." The whole of Ezek. xvi. must be read with Ex. xix.4-6. It is this "first love" which Israel is here charged with having "left." This is the beginning of all the subsequent evil.