Having made known unto us the mystery of his will. The wide extent of God's grace was a mystery, i.e. a hidden counsel, before Christ came and died, but it is now made known. In this, and not in the modern sense of mystery, the word μυστήριον is used by Paul.
The thing hidden and now revealed was not the gospel, but God's purpose with reference to its limits or sphere (see Ephesians 3:6). According to his good pleasure which he purposed in himself. The whole phraseology denotes that, in this transaction, God was not influenced by any external considerations; the whole reason for it sprang from within.