Commentaries:
Robertson's Word Pictures (NT)
What must we do? (Ti poiwmen). Present active deliberative subjunctive of poiew, "What are we to do as a habit?" For the aorist subjunctive (poihswmen) in a like question for a single act see Luke 3:10. For the present indicative (poioumen) of inquiry concerning actual conduct see John 11:47 (what are we doing?).
That we may work the works of God (ina ergazwmeqa ta erga tou qeou). Final clause with ina and the present middle subjunctive, "that we may go on working the works of God." There may have been an element of vague sincerity in this question in spite of their supercilious attitude.
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