Jamieson, Fausset and Brown on Ephesians 1:9

9. "He hath abounded," or "made (grace) to abound toward us" (Ep 1:8), in that He made known to us, namely, experimentally, in our hearts.

the mystery--God's purpose of redemption hidden heretofore in His counsels, but now revealed (Ep 6:19; Ro 16:25; Col 1:26, 27). This "mystery" is not like the heathen mysteries, which were imparted only to the initiated few. All Christians are the initiated. Only unbelievers are the uninitiated.

according to his good pleasure--showing the cause why "He hath made known to us the mystery," namely, His own loving "good pleasure" toward us; also the time and manner of His doing so, are according to His good pleasure.

purposed--(Ep 1:11).

in himself--God the Father. Bengel takes it, "in Him," that is, Christ, as in Ep 1:3, 4. But the proper name, "in Christ," Ep 1:10, immediately after, is inconsistent with His being here meant by the pronoun.