In Greek, this verse is a continuation of the sentence begun in verse 20 and consists of four separate clauses introduced in the following way: (1) that (Greek hina), (2) just as (Greek kathōs), (3) that (Greek hina), (4) that (Greek hina). Since Good News Translation has a full stop at the end of verse 20 and begins a new sentence with verse 21, it reintroduces the words I pray from verse 20. The first Greek clause is then represented by I pray that they may all be one; and because it is more natural in English to introduce the positive statement before the comparison, Good News Translation reorders the second and third clauses. It places a full stop at the end of the second clause also, making it necessary to introduce this clause with such a formula as “I pray” or “may.” For stylistic reasons, the verb may is used, indicating that this is a continuation of Jesus’ prayer of request to the Father. The last clause in Greek is literally “that the world may believe that you sent me,” which Good News Translation explicitly connects with the earlier petition, May they be one.
The purpose expressed in the clause that they may all be one may require modification, especially in languages in which a numeral such as one must show agreement with a subject. For example, in such languages, a plural affix added to a numeral such as one would be meaningless. However, one may sometimes introduce an expression which will result in a simile, for example, “I pray that they all may be just like one.” It may be possible, or even necessary, to be more specific in terms of action or behavior, for example, “I pray that they may act together just as though they were one person.”
Just as you are in me and I am in you develops the theme of unity between Father and Son mentioned in verse 11 (Just as you and I are one). The concept of one person in another is difficult, and it may be better to translate “May they be united with us, just as you are united with me and I am united with you” or “may they be united with us, just as you and I are united to each other.”
Good News Translation understands the clause at the end of verse 21 as expressing the purpose of the unity of future believers among themselves (May they be one, so that the world will believe that you sent me). Die Bibel im heutigen Deutsch has “Just as you are in me and I am in you, Father, so must they also become one through us! Then shall the world believe that you have sent me.” This translation is similar to Good News Translation. However, it includes the word “one” contained in the variant reading discussed below (“so must they also become one through us”) and translates the Greek preposition en “through” rather than in. Other translations make the last clause express the purpose of unity that future believers will have with the Father and the Son. New English Bible has “so also may they be in us, that the world may believe that thou didst send me”; Goodspeed translates “let them be in union with us, so that the world may believe that you sent me.” The Greek text permits either exegesis, although in the Greek sentence order May they be in us is closer to the purpose clause than that they all may be one. The difference in meaning is not great, since the unity of future believers among themselves is the result of their unity with the Father and the Son.
Instead of may they be in us, some Greek manuscripts have “that they may be one in us.” The word “one” is most probably a repetition of the same word earlier in the verse. It is often impossible to begin a sentence with such a request as May they be in us. Such a petition must be introduced by a specific term for prayer, for example, “I pray that they may be in us,” “that they may be united to us,” or “… be joined to us.” It may also be necessary to introduce the final petition, May they be one, by a verb, for example, “I pray that they may be one.”
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 .