Except that Paul also said that the “mystery” had been given to him that had not been known before. It was revealed by the Holy Spirit
Ephesians 1:1-5 For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles; if indeed you have heard of the dispensation of the grace of God which was given to me for you, how that by revelation He made known to me the mystery (as I have briefly written already, by which, when you read, you may understand my knowledge in the mystery of Christ), which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets:
If it had been a mystery until the Holy Spirit revealed it to them it had to have been still a “mystery” throughout Christ’s life on earth and until the apostles were given the indwelling of the Holy Spirit. Something changed at that point. That’s when grace took center stage.
He used "apostles" plural.