A.T. Robertson Commentary John 17:12

"While I was with them, I kept them in thy name which thou hast given me: and I guarded them, and not one of them perished, but the son of perdition; that the scripture might be fulfilled." — John 17:12 (ASV)

I kept (ετηρουν). Imperfect active of τηρεω, "I continued to keep."

I guarded (εφυλαξα). First aorist (constative) active of φυλασσω. Christ was the sentinel (φυλαξ, Acts 5:23) for them. Is he our sentinel now?

But the son of perdition (ε μη ο υιος της απωλειας). The very phrase for antichrist (2 Thessalonians 2:3). Note play on απωλετο, perished (second aorist middle indicative of απολλυμ). It means the son marked by final loss, not annihilation, but meeting one's destiny (Acts 2:25). A sad and terrible exception .

The scripture (η γραφη). It is not clear whether this is John's own comment or the word of Jesus. Not in 18:9. The Scripture referred to is probably Ps 41:9 quoted in 13:18 with the same formula ινα πληρωθη which see there.