Paul gives a powerful compliment, but it's laced with a critical observation. How can zeal for God be wrong?
Paul starts by acknowledging the intense passion of the Jewish people. He 'bears witness'—meaning he is a genuine witness, speaking from personal experience—that they have 'a zeal for God.' This wasn't a fake or lukewarm devotion; it was passionate, all-consuming. Think of Paul himself before his conversion, fiercely persecuting Christians, believing he was serving God (Acts 26:9-11). He recognized their sincerity and commitment.
However, he immediately pivots. This zeal, while real, wasn't 'according to knowledge.' This means their passion wasn't guided by a true, full understanding of God. Their actions, though motivated by a desire to honor God, were based on flawed information and misconceptions about who God is and what He requires. It's like having a powerful engine but no steering wheel – lots of energy, but going in the wrong direction.