Ephesians 3:3

how that by revelation the secret was made known to me, as I wrote before in few words,
- Ephesians 3:3

The false teachers were saying that the apostles did not have the full revelation of God like the prophets did.  Since Paul was an apostle, it was said that his message of salvation was incomplete and that he did not know that the Spirit of Christ only offered spiritual salvation.  The false teachers considered spiritual salvation to be secret knowledge.

Paul responds by saying that he had received revelation just like the prophets (see Acts 26) and that the secret was made known to him as he had written them before.  Although we don't know what Paul had written before, we know from Ephesians, that the "secret" is Christ and that He provides salvation for all.