Have I known; i.e. loved, acknowledged, chosen. So in Hosea 13:5 God says. "I knew thee in the wilderness;" and St. Paul (2 Timothy 2:19), "The Lord knoweth them that are his" (comp. Nahum 1:7). The peculiar relation in which God allowed Israel to stand to him is much dwelt upon (see Deuteronomy 4:8, Deuteronomy 4:20; Deuteronomy 14:2; 2 Samuel 7:23; 1 Chronicles 17:21).
Therefore I will punish you; literally, visit upon you. They must not presume upon their privileges; the retention of God's favour depended upon obedience to his Word (Exodus 19:5): the nearer they were brought to God, the greater their guilt if they fell from him.
Unlike the nations denounced in the former chapters, Israel had sinned against light and knowledge and love, therefore the sentence on her must be heavier (comp. Ezekiel 9:6; Luke 12:47; 1 Peter 4:17).