I know Ephraim, and Israel is not hid from me: for now, O Ephraim, thou committest whoredom, and Israel is defiled.
I know Ephraim - the tribe so called, as distinguished from "Israel" here the other nine tribes. It was always foremost of the tribes of the north kingdom. For 400 years in early history it, with Manasseh and Benjamin, its two dependent tribes, held the preeminence in the whole nation. Ephraim is here addressed as foremost in idolatry.
And Israel is not hid from me - notwithstanding their supposed profound cunning (Hosea 5:2; Revelation 2:2; Revelation 2:9; Revelation 2:13; Revelation 2:19, "I know thy works").
For now, O Ephraim, thou committest whoredom - "now," "though I have been a rebuker of all them" (Hosea 5:2) who commit such spiritual whoredoms, thou art now continuing in them. Now, when thou thinkest thy doings are profoundly hidden (Hosea 5:2), I know them all and thyself.