For I bear them record that they have a zeal of God, but not according to knowledge.
For I bear them record (or, 'witness') - as well he could from his own sad experience,
That they have a zeal of ('for') God, but not according to knowledge (cf. Acts 22:3; Acts 26:9-11; Galatians 1:13-14). He alludes to this well-meaning of his people, notwithstanding their spiritual blindness, not certainly to excuse their rejection of Christ and rage against His saints, but as some ground of hope regarding them (see 1 Timothy 1:13).
Self-righteousness was the fatal rock on which Israel was split-Christ the divinely-provided, divinely-predicted, only, and all-sufficient righteousness of the sinner, whether Jew or Gentile, that believeth (Romans 10:3-13)