What does John 17:8 mean?
This echoes Jesus' earlier statements: that part of His mission was to pass along the message of God the Father (John 8:28; 12:49). As He prays for the disciples, in particular, He refers to their faith.Jesus' closest followers were far from perfect. He spent quite a bit of time correcting—even rebuking—when they struggled to understand His lessons (Matthew 8:26). Sometimes, those mistakes were a matter of misinterpreting the Old Testament, or the law of Moses (Luke 24:25–26). In other instances, pride and overconfidence tripped them up (Mark 8:33). Despite those errors, the men Jesus selected to become "the Twelve" (Matthew 10:1–4) held sincere, legitimate faith in Him and His message (John 6:68–69). That has always been the one and only requirement to be a "true believer." Perfection is not required—only humble, submissive faith.
Of course, one of the men Jesus placed in His inner circle chose betrayal and damnation, instead. Judas' treachery wasn't a surprise to Christ (John 6:64), and it's something still on His mind even as He prays for those who remained loyal (John 17:12).