Only (μονον). One item was emphasized.

We should remember (μνημονευωμεν). Present active subjunctive, "that we should keep on remembering."

Which very thing (ο--αυτο τουτο). Repetition of relative and demonstrative, tautology, "which this very thing." In fact Barnabas and Saul had done it before (Acts 11:30). It was complete victory for Paul and Barnabas. Paul passes by the second public meeting and the letters to Antioch (Acts 15:6-29) and passes on to Peter's conduct in Antioch.