The relation of this verse to the context is not at all clear. Some believe it is a parenthetical insertion, since the slave of verse 34 is quite different from the slave mentioned in this verse. New American Bible places it in curved brackets, indicating that it is a parenthetical statement. However, it is possible to see a relation between this verse and the total context. In verse 33 a discussion regarding the descendants of Abraham was initiated, and this recalls Genesis 21.10 (see Gal 4.30). Isaac remains a member of the household, while Ishmael, who was born of a slave woman, is driven out. The Jews claim to be the true sons of Abraham, while in fact they are the sons of sin, and so have lost their status as sons. In this brief parable, then, the Jews are spoken of as a slave, while Christ is referred to as the son, as elsewhere in John’s Gospel. By their rejection of the son, the Jews have lost their status in God’s household, but the son will remain there forever.
The verb belong to is literally “remains in,” the same verb discussed in verse 31.
In some languages it may not make sense to say A slave does not belong to a family. He has his own family, of course, but what is involved here is the household he serves. Therefore one may translate “A slave does not continue as a permanent member of a household” or “… may not necessarily continue as a member of a household.”
The final clause may then be translated “but a son is always a member of such a household.” In some languages, however, it is not possible to say “a son.” Such a person must be in a possessed relationship to someone. Therefore one may have to say “a man’s son always belongs to the father’s household” or “… the father’s family” or in some languages, “… his parents’ family.” There are, of course, certain problems involved in such expressions, particularly in matrilineal societies, where it would be necessary to speak of “a son belonging to his mother’s family.”
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 .