Charles Ellicott Commentary John 18:9

"that the word might be fulfilled which he spake, Of those whom thou hast given me I lost not one." — John 18:9 (ASV)

That the saying might be fulfilled, which he spake. . The quotation is in many ways suggestive:

1. It is not verbally accurate; i.e., St. John, quoting the words of Christ, which he himself has recorded a few verses before, takes no pains to reproduce it word for word, but is satisfied with giving its substance. This practice throws light on the general literary habits and feelings of this era and people, and it is in full harmony with the usual practice of quotation in the New Testament.
2. St. John quotes with an application to temporal persecution what had been spoken of spiritual persecution. This illustrates the way in which words are said to be “fulfilled” in more than one sense. Striking words become fixed in the mind, and when an event occurs that illustrates their meaning, it is therefore said to fulfill them, although each fulfillment can be only a part. (Compare especially Notes on John 2:17 and John 12:38 and following).
3. The quotation shows that in St. John's own thought, the prayer recorded in John 17 is not a résumé of the words of our Lord, but an actual record of His prayer; he quotes the “saying” as fulfilled, just as he would have quoted a passage from the Old Testament Scriptures.