As indicated under 1.14, “truth” is not an abstract idea in biblical thought. When the statement is made that God is true, it means basically that God is faithful. Accordingly, when the writer here speaks of persons who do what is true, he refers to those who continue in faithful obedience to God. Specifically, in the context of this verse, the idea is that of responding faithfully to the revelation of light that God has given concerning himself.
In some languages it makes little sense to say “to do what is true,” for “truth” would relate only to some verbal formulation or statement and would not be applicable to a particular kind of deed. In such instances it is better to base a translation upon the meaning of this expression, for example, “whoever does what God says is right” or, better “whoever obeys the truth, that is, God himself” or simply “whoever obeys God, who is true.”
In some instances comes to the light may be more appropriately interpreted as “exposes himself to the light” or “lets the light shine on him.” This meaning is more adequately expressed in some languages as “lets everyone see what he is doing.” In this context, “light” is not to be interpreted abstractly, but as providing the means by which a man’s deeds may be seen. This meaning is made particularly clear in the second half of this verse.
In order that the light may show that what he did was in obedience to God is literally “in order that it might be shown that his works have been worked in God.” Good News Translation makes explicit the agent of the passive verb “that it might be shown” and changes to an active construction, that the light may show. The idea of “doing work in God” sounds to the English reader like a spatial relationship. Good News Translation avoids this idea by the rendering that what he did was in obedience to God. New English Bible has “That God is in all he does.” The idea either of obedience to God or “union with God” is a legitimate interpretation of this verse, but such pitfalls as the rendering found in Jerusalem Bible, “what he does is done in God” or Revised Standard Version, “his deeds have been wrought in God,” should be avoided, since the phrase “in God” is at best a zero meaning for the average reader of English.
What he did was in obedience to God must undergo a certain amount of restructuring in some languages, for example, “he did what he did because he was obedient to God” or “what he did showed that he was obeying God.”
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 .