I knew. Put by Figure of speech Metonymy (of Cause), for choosing. Reference to Pentateuch (Exodus 33:12; Exodus 33:17). App-92.
sanctified thee. set thee apart, or, hallowed thee. See note on Exodus 3:5, and compare John Baptist (Luke 1:15-17); Paul (Galatians 1:15; Galatians 1:16); Samson (Judges 13:3).
the nations. This distinguishes Jeremiah from some of the other prophets, and shows that the legend of his martyrdom is only legend.