Vincent's NT Word Studies
9. Having made known. The participle is explanatory of which He made to abound, etc.: in that He made known.The mystery of His will. For mystery, see on Rom. xi. 25; Colossians i. 26. Another key-word of this epistle. God's grace as manifested in redemption is a mystery in virtue of its riches and depth - as the expression of God's very nature. The mystery of the redemption in Christ, belonging to the eternal plan of God, could be known to men only through revelation - making known. Of his will; pertaining to his will. Compare ch. iii. 9.
Purposed (proeqeto). Only here, Rom. i. 13; iii. 25 (note).
In Himself (en autw). The best texts read aujtw in Him; but the reference is clearly to God, not to Christ, who is expressly mentioned in the next verse.
Robertson's NT Word Studies
1:9 {The mystery of his will} (to musterion tou qelematos autou). Once hidden, now revealed as in #Col 1:26 which see. See also #Col 2:3. {Which he purposed} (hen proeqeto). Second aorist middle of protiqemi, old verb, for which see #Ro 1:13; 3:25.