‘For indeed you are doing it towards all the brothers and sisters who are in all Macedonia.'
What a testimony was that. The Thessalonians were famed for their true and continuing love of their fellow believers. That is why Paul felt that he had no need to mention it to them (although he did).
‘But we exhort you, brothers and sisters, that you abound more and more.'
While their love is so well known it has not reached perfection, and therefore he exhorts them not to be content but to let it grow and grow, by more and more self-giving. We can compare here Philippians 1:9-11 where Paul prays that love may abound yet more and more ‘in all spiritual knowledge (epignosis) and discernment'. Love to be true love must be in line with the highest ideals, and those result from spiritual knowledge and discernment.