Charles Hodge on Ephesians 1:9

V. 9. God has caused this wisdom to abound, or has communicated it, having made known unto us the mystery of his will, γνωρίσας ἡμῖν τὸ μυστήριον τοῦ θελήματος αὐτοῦ. In other words, by the revelation of the Gospel. The word μυστήριον, mystery, means a secret, something into which we must be initiated; something, which being undiscoverable by us, can be known only as it is revealed. In this sense the Gospel is a mystery; and any fact or truth, however simple in itself, in the New Testament sense of the word, is a mystery, if it lies beyond the reach of our powers. Comp. Rom. 16, 25. 1 Cor. 2, 7-10. Eph. 3, 9. Col. 1, 26. For the same reason any doctrine imperfectly revealed is a mystery. It remains in a measure secret. Thus in the fifth chapter of this epistle Paul calls the union of Christ and believers a great mystery; and in 1 Tim. 3, 16 he calls the manifestation of God in the flesh, the great mystery of godliness.

In the present case the mystery of his will means his secret purpose; that purpose of redemption, which having been hid for ages, he has now graciously revealed.

According to his good pleasure, κατὰ τὴν εὐδοκίαν αὐτοῦ, ἣν προέθετο ἐν αὐτῷ. There are three interpretations of this clause. The first is to make it qualify the word will. ‘His will which was according to his good pleasure;’ i. e. his kind and sovereign will. But this is forbidden by the absence of the connecting article in the Greek, and also by the following clause. The second interpretation connects this clause with the beginning of the verse, ‘Having, according to his good pleasure, made known the mystery of his will.’ The sense in this case is good, but this interpretation supposes the relative which, in the following clause, to refer to the mystery of his will, which its grammatical form in the Greek forbids. Which (ἣν) must refer to good pleasure (εὐδοκία). The third explanation, which alone seems consistent with the context, supposes εὐδοκία to mean here not benevolence, but kind intention, or, sovereign purpose. The sense then is: ‘Having made known the mystery of his will, according to his kind intention or purpose (viz. of redemption) which he had purposed in himself.’ Instead of in himself, many commentators read in him, referring to Christ. But this would introduce tautology into the passage. The apostle would then say: ‘Which he purposed in Christ, to bring together in Christ.’