I am glad of the coming of Stephanas and Fortunatus and Achaicus: for that which was lacking on your part they have supplied. I am glad of the coming of Stephanas, etc. - It was by these that the Corinthians had sent that letter to the apostle, to answer which was a main part of the design of St. Paul in this epistle.
Fortunatus - This man is supposed to have survived St. Paul; and to be the same mentioned by Clement in his epistle to the Corinthians, sec. 59, as the bearer of that epistle from Clement at Rome to the Christians at Corinth.
For that which was lacking on your part - This may either refer to additional communications besides those contained in the letter which the Corinthians sent to the apostle - which additional circumstances were furnished by the persons above; and from them St. Paul had a fuller account of their spiritual state than was contained in the letter - or to some contributions on their part for the support of the apostle in his peregrinations and labors.