Verily, verily. This impressive recommencement of discourse implies that a new turn is given to the conversation, and that the gravest solemnity and importance is attached to the utterance. It is impossible that the Jews should have listened unmoved to Christ's rejoinder on their rude taunt, or been unimpressed by the self-composed and lofty way in which the honour of our Lord was calmly entrusted by him to the Father.
The Jews may say what they please, call him by any opprobrious name they choose; "there is One that seeketh" his glory, and he is content. He has, in earlier portions of this discourse, promised freedom and sonship to those who abide in his word; and now to those who believed on him he says, with extraordinary emphasis, If a man (any one) have kept my word, he shall never behold death.
This "keeping" is more than "abiding" in the word. There is the additional notion of intently watching the "keeping," which issues in "fulfilling" and "obeying" (Meyer and Tholuck); see John 8:55; John 14:15, John 14:21, John 14:23; John 15:20; John 17:6.
The opposite of τηρεῖν would be "to disregard;" the opposite of φυλάσσειν would be "to let slip" (Westcott). The promise is dazzling: "He shall never behold," i.e. steadily or exhaustively know by experience, what death means and is.
He may pass through physical death, he may ( γεύσηται) taste of dissolution, he may come before the judgment seat, he may see corruption ( ἰδεῖν διαφθοράν); but he will not behold ( θεωρεῖν) death.
He will never know what death is (cf. here; John 4:14; John 5:24; John 6:51, where the Saviour speaks of the "living water," and "life eternal," and "living bread," which whoso partaketh shall never die..
See also John 11:26). He does not tell his disciples that they shall not see the grave, but that in the deepest sense they shall never die. "Death" and "life" are words that are lifted into a higher connotation.
Death is a moral state, not an event in their physical existence.