# I know Ephraim, and Israel is not hidden from me
Here, "Ephraim" and "Israel" both refer to the whole northern kingdom of Israel, and represent the people who live there. Here God says that he knows what they are like and what they are doing.
# Ephraim, now you have become like a prostitute
Ephraim is presented in terms of a prostitute because the people have become unfaithful to God, as a prostitute is faithful to no man.