For yourselves know (autoi gar oidate). This explanatory gar takes up in verses Acts 1-12 the allusion in Acts 1:9 about the "report" concerning the entrance (eisodon, way in, ei, odon), unto you (thn pro uma). Note repeated article to sharpen the point. This proleptic accusative is common enough. It is expanded by the epexegetic use of the oti clause that it hath not been found vain (oti ou kenh gegonen). Literally, that it has not become empty. Second perfect active (completed state) of ginomai. Every pastor watches wistfully to see what will be the outcome of his work. Bengel says: Non inanis, sed plena virtutis. Cf. Acts 1:5 . Keno is hollow, empty, while mataio is fruitless, ineffective. In 1 Corinthians 15:141 Corinthians 15:17 Paul speaks of kenon to khrugma (empty the preaching) and mataia h pisti (vain the faith). One easily leads to the other.