In So we ought to support, the particle So introduces the conclusion based on what precedes. The same is the case in verse 10.
We is emphatic and contrasts with the heathen. The intention is “if the preachers accept nothing from the heathen, it is clearly our duty to provide for them”; hence “we Christians” (Good News Translation). For ought see comments on 1 John 2.6.
To support: the Greek verb means “to welcome as guests,” then generally “to support,” “to give help and assistance to.” Both meanings are defensible here, but the second fits best because of verse 6b.
That may refer here either to purpose, “in order that,” or to expected result, “so that.”
We may be fellow workers in the truth: the Greek noun rendered fellow workers literally means “those-who-are-working-together,” “those-who-are cooperating (or helping/assisting).” This word is followed here by the noun truth. In the Greek this noun is in a form that here serves to mark the person with whom one is cooperating. This implies that the truth (in the sense of “God’s truthfulness,” compare comments on 1 John 1.8) functions here as a personified quality of God, with which the traveling preachers are working together. sunergos.
In the opinion of the present authors, this interpretation best fits the construction used in the Greek. It is represented in such renderings of the clause as ‘we may be cooperating with (or be helpers of) the truth,’ ‘we may work together with (or join in the work of) the truthful One.’
Many versions, however, perhaps even the majority, follow another interpretation. They take truth in the sense of “the cause of truth” or “the true cause,” indicating the field in which, or the aim for which, we are to work. Then the persons with whom they are to cooperate are the traveling preachers and must be understood from the context.
This interpretation has led to renderings like “we may share in their (that is, the preachers’) work for the truth” (Good News Translation, compare also Bible de Jérusalem), ‘we may unite with them as they proclaim the truth,’ ‘we as well as they may spread the true work,’ ‘we will help them in (their work of) teaching true things.’
Quoted with permission from Haas, C., de Jonge, M. and Swellengrebel, J.L. A Handbook on The Third Letter of John. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .
