Theophylact of Ohrid

That is, He poured out grace upon us so that, having made us wise and understanding, He might "reveal to us the mystery of His will"; he was saying, as it were: He revealed to us the mysteries of His heart. Or in this way: "to show us the mystery in all understanding and wisdom," that is, the depth of all wisdom and understanding. For to deem enemies, and despised ones at that, worthy of such great blessings, and this at the end of times and through the cross — is this not a work of great wisdom? …

Commentary on Ephesians ↗

Cosmas Indicopleustes

According to his good pleasure, he says, which he purposed in him; that is, He longed for this, He was in throes, as one may say, to bring forth this mystery. And of what nature is this? It is that it is His will to set man on high, and this He desires with a view to a dispensation of the fulness of the times to sum up all things in Christ, the things in the heavens, and the things upon the earth.

The Christian Topography, Book 10 ↗

Jerome

Some attentive reader might object: "If Paul knows in part and prophesies in part and now sees as through a glass darkly, how is the mystery of God revealed either to him or to the Ephesians "in all wisdom and insight?" … It is not that they by themselves have learned this mystery "in all wisdom and insight," but God "in all wisdom and insight" has revealed the mystery to us, so far as we are able to grasp it.

Commentary on Ephesians 1:9 ↗

Thomas Aquinas

The reception of an uncommon revelation is their next blessing, that he might make known unto us the mystery of his will. As if he had said: Our wisdom does not consist in discovering the natures of material realities, nor the course of the stars, or such like; rather, it concerns Christ alone. "I decided not to know any thing among you, but Jesus Christ, and him crucified" (1 Cor. 2:2). Hence he says that he might make known the mystery, that is, the sacred secret, hidden from the beginning, the mystery of the Incarnation. …

Commentary on Ephesians ↗