You only have I known - that is, acknowledged as My people, and treated with peculiar favor (Exo 19:5; Deu 4:20). Compare the use of “know,” Psa 1:6; Psa 144:3; Joh 10:14; 2Ti 2:19.
therefore I will punish - the greater the privileges, the heavier the punishment for the abuse of them; for to the other offenses there is added, in this case, ingratitude. When God’s people do not glorify Him, He glorifies Himself by punishing them.