John Chrysostom

"According to the eternal purpose." It hath been now, he means, brought to pass, but not now decreed, it had been planned beforehand from the very first. "According to the eternal purpose which He purposed in Christ Jesus our Lord." That is, according to the eternal foreknowledge; foreknowing the things to come, i.e., he means the ages to come; for He knew what was to be, and thus decreed it. According to the purpose of the ages, of those, perhaps, which He hath made by Christ Jesus, because it was by Christ that every thing was made.

Homily on Ephesians 7 ↗

Theophylact of Ohrid

Although the wisdom of what has been accomplished with us has only now been made known, he says, it was predetermined from of old. "According to the eternal purpose," that is, according to the foreknowledge of the ages to come. For God knew the future and thus predetermined it. The expression "which He accomplished in Christ Jesus" refers to the wisdom or to the dispensation which the Father accomplished through the Son. …

Commentary on Ephesians ↗

Thomas Aquinas

The way this revealed knowledge is many-sided is according to a pre-determining of the ages, meaning the differentiation and limitation of the various times. For God plans something to exist at one time, and others at another time. In this fashion such wisdom is referred to as manifold according to a pre-determining of the ages since he provides different times with different events. The source of this multiplicity is Christ; hence he says which God made in Christ Jesus our Lord, that is, through Christ. …

Commentary on Ephesians ↗