John Chrysostom

"Peace," saith he, that "peace" which is towards God. He hath reconciled us. For the Lord Himself also saith, "Peace I leave with you; my peace I give unto you." And again, "Be of good cheer, I have overcome the world." And again, "Whatsoever ye shall ask in my name that will I do." And again, "For the Father loveth you." These are so many evidences of peace. But how towards the Gentiles? "Because through Him we both have our access in one Spirit unto the Father," not ye less, and they more, but all by one and the same grace. …

Homily on Ephesians 6 ↗

Theophylact of Ohrid

By His death He destroyed the enmity and then made us beloved to the Father through the Spirit, having graced us with the same grace. For He did not grant them more, as to those who were near, and us less, as to those who were far off; but in one Spirit He granted to both the same grace and thus brought them to the Father. Or "in" is used instead of "through," and then the meaning would be this: through Him and the Spirit we have been brought to the Father.

Commentary on Ephesians ↗

Jerome

However, it should not be thought possible to achieve perfect and complete reconciliation in this world.… The making of the new person in Christ will be fully consummated when earthly and heavenly things have been reconciled, when we come to the Father in one Spirit and with one affection and understanding.

Commentary on Ephesians 2:1-5 ↗

Thomas Aquinas

He indicates the cause and form of peace by saying "For by him we have access both," that is, the two peoples, "in one Spirit," meaning we are joined by the union of the Holy Spirit. "Careful to keep the unity of the Spirit in the bond of peace" (Eph 4:3). "One and the same Spirit produces all these" (1 Cor 12:11). The way we enjoy access to the Father is through Christ since Christ works through the Holy Spirit. "Now if anyone does not have the Spirit of Christ, he does not belong to him" (Rom 8:9). …

Commentary on Ephesians ↗