By referring to this, when you read it you can understand my insight into the mystery of Christ, which in other generations was not disclosed to mankind, as it has now been revealed to His holy apostles and prophets by the [Holy] Spirit; [it is this:] that the Gentiles are now joint heirs [with the Jews] and members of the same body, and joint partakers [sharing] in the [same divine] promise in Christ Jesus through [their faith in] the good news [of salvation]. Of this [gospel] I was made a minister by the gift of God’s grace given me through the working of His power. To me, [though I am] the very least of all the saints (God’s people), this grace [which is undeserved] was graciously given, to proclaim to the Gentiles the good news of the incomprehensible riches of Christ [that spiritual wealth which no one can fully understand], and to make plain [to everyone] the plan of the mystery [regarding the uniting of believing Jews and Gentiles into one body] which [until now] was kept hidden through the ages in [the mind of] God who created all things. – Ephesians 3:4-9
That the calling of the Gentiles was made known by the prophets in different ages of the Jewish Church is exceedingly clear; but it certainly was not made known in that clear and precise manner in which it was now revealed by the Spirit unto the ministers of the New Testament: nor was it made known unto them at all, that the Gentiles should find salvation without coming under the yoke of the Mosaic law, and that the Jews themselves should be freed from that yoke of bondage; these were discoveries totally new, and now revealed for the first time by the Spirit of God. – Adam Clarke
The apostles and prophets were the superior officers in the Gospel dispensation; the former design the twelve apostles of Christ, and the latter such who had the gift of interpreting the prophecies of the Old Testament, and of foretelling things to come, having received gifts from Christ to fit them for such offices, some apostles, some prophets; and to these a revelation was made of the mystery of the Gospel in general, and of the calling of the Gentiles in particular, by the Spirit, who searches the deep things of God, and reveals them, and leads into all truth; and who, by falling upon the Gentiles, as upon Cornelius and his family, and by the success which he gave to the Gospel in the Gentile world, made their calling clear and manifest. – John Gill
Dunamai means to have power by virtue of inherent ability and resources and so to be able. In context do you see what Paul is saying? We would not be able to understand the supernatural Word without the Spirit’s supernatural enablement, His dunamis, His enabling power! The Spirit of Truth takes the Word of Truth and daily (if we go to His Word daily) opens our “minds to understand (suniemi) the Scriptures” just as Jesus did for His first disciples.