On his own authority is the same expression translated on my own authority in the previous verse, except that there the third person is used in place of the first. Sometimes it is difficult to render the expression speaks on his own authority. It may be necessary to say “speaks, claiming that his words are just his own” or “speaks, saying that his words are his own thoughts alone” or “… saying that he himself has thought up all that he says” or “speaks only from his own mind.”
The meaning of is trying to gain glory for himself is clear, though the Greek underlying this phrase is translated in a variety of ways. The contrast is between a teacher who represents himself as the source of knowledge and one who speaks as the representative of another from whom he has received his knowledge. Jerusalem Bible translates “is hoping to get honour for himself,” and New English Bible has “aims at honour for himself.” Phillips is rather high level, “has an eye for his own reputation.” Moffatt translates “aims at his own credit.” To gain glory for himself may be expressed idiomatically in some languages as “to raise up his own name,” “to make everyone shout his name” or “to make everyone say of him, Isn’t he wonderful?”
He who wants glory for the one who sent him may be rendered “he who wants others to praise the one who sent him” or “he who wants the person who sent him to be honored.”
It is often necessary to place an adversative expression (English “but” or “however”) at the beginning of the second sentence of verse 18, for example, “but he who wants people to honor the one who sent him.” It is also possible to use a conditional expression, for example, “If anyone wants people to honor the one who sent him, then that one is honest.” The difficulty with the conditional expression is that it makes the reference too broad; as a relative clause it is possible to make the reference more specifically applicable to Jesus.
The word translated honest (Greek alēthēs; Revised Standard Version “true”; New American Bible “truthful”; Jerusalem Bible, New English Bible “sincere”) is used of God in 3.33 and in 8.26; in both cases it is translated true in Good News Translation. In the present context honest is used of one who speaks what is true. The word translated false (so also New English Bible; Revised Standard Version “falsehood”; Moffatt, New American Bible “dishonesty”) is used only here in the Gospel of John. It is used many times in the Septuagint to translate the Hebrew word meaning “lie.” In this verse it is obvious that honest and false are used in contrast to each other. This contrast may be expressed in some languages as “he tells the truth; he doesn’t lie at all” or “he tells what is true; not at all does he tell what is not true” or “… false.”
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 .