In the Son is literally “in him,” but the pronominal reference is made explicit in order not to confuse “him” with God, also mentioned in the previous verse.

It is important to note the verb tenses used in this verse. The verb believes and the verb phrase is not judged are both in the present tense and so indicate that the believer is no longer under God’s judgement. The expression does not believe is also in the present tense, but it is followed by two verbs in the perfect tense (already been judged … has not believed). The force of this construction is to emphasize the continuing effect of God’s judgement on those who have refused to believe. That is, according to John’s Gospel, eternal life is not only a hope for the future, but also a present reality; and so also God’s judgement on a man’s life begins here and now, whenever a man refuses to be obedient to God.

In God’s only Son translates “in the name of God’s only Son.” See the discussions at 1.14 and 14.13 for “only” and “name.” Jerusalem Bible has a note: “Semitism: the ‘name’ is the person.”

In some languages it is necessary to introduce the agent of judging. It may also be necessary to change the indefinite relative clause whoever believes in the Son into a conditional; for example, “If anyone believes in the Son, God does not condemn him.” This sentence may also be restructured: “God does not condemn those who believe in his Son.” The following clause may then be made parallel; for example, “but God has already condemned those who do not believe in his Son.” Such a translation tends to make superfluous the final clause because he has not believed in God’s only Son, but this clause may be made emphatic by rendering “he condemns him precisely because he has not believed in his only Son.”

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 .