I am telling you the truth is the same formula that was first used in 1.51 (see there), except that you is singular in 21.18.

The verbs rendered you used to get ready and go are in the imperfect tense in Greek, expressing customary action in the past. New American Bible renders “as a young man you fastened your belt and went about as you pleased,” and New American Bible has “when you were young you fastened your belt about you and walked where you chose.” The Greek verb rendered “fastened your belt” expresses the action of putting a girdle around one’s loose, flowing robes before going out into the street or embarking on a journey. Since this practice is unknown to the English reader, Good News Translation renders you used to get ready.

The second part of this verse is more difficult. On the basis of verse 19 it is clear that the reference is to Peter’s death as a Christian martyr. Since the earliest tradition is that Peter was crucified, a number of commentators take you will stretch out your hands as an explicit reference to his crucifixion. Someone else will tie you up would then refer to the binding of a prisoner to lead him off to crucifixion. If this interpretation is valid, the fact that the stretching out of the hands is mentioned first, even though the tying up would be first in chronological order, is explained by the fact that the reference to crucifixion was the central point of the saying. New American Bible says in a note that this verse is “A figurative reference to the crucifixion of Peter.”

Some have argued that verse 18b cannot refer to Peter’s martyrdom, since the verb rendered tie … up (the same verb translated get ready in the first half of the verse) is never used in Greek of tying up a prisoner. If verse 18b cannot refer to Peter’s martyrdom, then, it is suggested, the contrast in the verse is between the alertness of Peter’s youth and the helplessness of his old age; as a young man he was able to get himself ready to go wherever he wished, but when he grows old, he will have to stretch out his hands for someone else to help him dress, and he will be taken where he does not want to go. This interpretation, however, cannot be reconciled with the Gospel writer’s explanation in the following verse: In saying this, Jesus was indicating the way in which Peter would die and bring glory to God.

An intermediate position would be to admit on the basis of verse 19 that verse 18b refers to Peter’s martyrdom but to leave open the question whether an explicit reference to crucifixion is intended by you will stretch out your hands. In an interpretation of verse 18b as a reference to martyrdom the use of zōnnumi (“fasten one’s belt”) to refer to tying up a prisoner would be an extension of its original meaning, allowable for the sake of the parallelism with verse 18a.

A number of translators (Revised Standard Version, Die Bibel im heutigen Deutsch, Moffatt, Goodspeed, La Sainte Bible: Nouvelle version Segond révisée, Luther, Zürcher Bibel, Phillips) keep the literal sense of “gird” or “dress” in verse 18b. However, others translate on the basis of the context: New American Bible “tie … fast”; New English Bible “bind … fast”; Barclay “bind.”

In some manuscripts the plural “others” is used instead of the singular (someone else). This change may have been made so that the prediction of Peter’s arrest corresponds with the arrest of Jesus, where the plural is used (18.12). A plural may be preferable in 21.18 on translational grounds, but it may be better in some languages to use a passive construction: “you will be taken where you don’t want to go.” One may also say “people will lead you off where you do not want to go” or “… will cause you to go where you do not want to go.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .