Charles Hodge on Ephesians 3:3

V. 3. How that by revelation was made known unto me, &c. This clause is connected with what precedes and explains it.—‘Ye have heard of the grace which I have received, i. e. ye have heard how that by revelation was made known to me.’ Κατὰ ἀποκάλυψιν, after the manner of a revelation, i. e. δι᾽ ἀποκαλύψεως, Gal. 1, 12. He was not indebted for his knowledge of the Gospel to the instructions of others, as he proves in his epistle to the Galatians by a long induction of facts in his history. This was one of the indispensable qualifications for the apostleship. As the apostles were witnesses, their knowledge must be direct and not founded on hearsay. The thing made known was a "mystery;" i. e. a secret, something undiscoverable by human reason, the knowledge of which could only be attained by revelation. This revelation was a grace or favour conferred on the apostle himself.

The mystery of which he here speaks is that of which the preceding chapters treat, viz. the union of the Gentiles with the Jews. Of that subject he had just written briefly; ἐν ὀλίγῳ, with little, i. e. few words.