The Lord … that pleadeth the cause of his people (comp. Jeremiah 50:34, which contains an allusion to this passage). As his people have a relentless adversary, who accuses them continually, and pleads against them (Revelation 12:10), so it is needful that they should have an untiring advocate.
God himself is this Advocate. The dregs of the cup (see the comment on Isaiah 51:17, ad fin.).