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 11:25; see on Col 1:26. 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 Eph 3:9.
Purposed (προέθετο)
Only here, Rom 1:13; Rom 3:25 (note).
In Himself (ἐν αὑτῷ)
The best texts read αὐτῷ in Him; but the reference is clearly to God, not to Christ, who is expressly mentioned in the next verse.