Ought (οφειλομεν). See for this word 1. John 2:6; 1. John 3:16; 1. John 4:11.

To welcome (υπολαμβανειν). Present active infinitive (habit of welcoming) of υπολαμβανω, old word, to take up under, to carry off (Acts 1:9), to reply (Luke 10:30), to suppose (Acts 2:15), only here in N.T. in this sense of receiving hospitably or to take under one's protection like υποδεχομα (Luke 10:38).

Such (τους τοιουτους). "The such" according to the Greek idiom (1. Corinthians 16:16; 1. Corinthians 16:18).

That we may be (ινα γινωμεθα). Purpose clause with ινα and the present middle subjunctive of γινομα, "that we may keep on becoming."

Fellow-workers (συνεργο). Old compound (συν, εργον).

With the truth (τη αληθεια). So associative instrumental case with συν in συνεργο, but it is not certain that this is the idea, though συνεργεω is so used with εργοις in James 2:22. Συνεργος itself occurs with the genitive of the person as in θεου συνεργο (1. Corinthians 3:9) or with genitive of the thing της χαρας (1. Corinthians 3:9). So then here the meaning may be either "co-workers with such brethren for the truth" (dative of advantage) or "co-workers with the truth" (associative instrumental case).