I kept (etēroun). Imperfect active of tēreō, “I continued to keep.”
I guarded (ephulaxa). First aorist (constative) active of phulassō. Christ was the sentinel (phulax, Act 5:23) for them. Is he our sentinel now?
But the son of perdition (ei mē ho huios tēs apōleias). The very phrase for antichrist (2Th 2:3). Note play on apōleto, perished (second aorist middle indicative of apollumi). It means the son marked by final loss, not annihilation, but meeting one’s destiny (Act 2:25). A sad and terrible exception (Mar 14:21).
The scripture (hē graphē). It is not clear whether this is John’s own comment or the word of Jesus. Not in Joh 18:9. The Scripture referred to is probably Psa 41:9 quoted in Joh 13:18 with the same formula hina plērōthēi which see there.