39 He shall deliver the Kingdom to God the Father, not in the sense that He resigns His power by the delivering, but that we, being conformed to the glory of His body, shall form the Kingdom of God. It is not said, He shall deliver up His Kingdom, but, He shall deliver up the Kingdom, that is, deliver up to God us who have been made the Kingdom by the glorifying of His body. He shall deliver us into the Kingdom, as it is said in the Gospel, Come, you blessed of My Father, inherit the Kingdom prepared for you from the foundation of the world. The just shall shine like the sun in the Kingdom of their Father, and the Son shall deliver to the Father, as His Kingdom, those whom He has called into His Kingdom, to whom also He has promised the blessedness of this Mystery, Blessed are the pure in heart, for they shall see God. While He reigns, He shall remove all things that cause stumbling, and then the just shall shine as the sun in the Kingdom of the Father.
Afterwards He shall deliver the Kingdom to the Father, and those whom He has handed to the Father, as the Kingdom, shall see God. He Himself witnesses to the Apostles what manner of Kingdom this is: The Kingdom of God is within you. Thus it is as King that He shall deliver up the Kingdom, and if any ask Who it is that delivers up the Kingdom, let him hear, Christ is risen from the dead, the firstfruits of them that sleep; since by man came death, by man came also the resurrection of the dead. All that is said on the point before us concerns the Mystery of the body, since Christ is the firstfruits of the dead.
Let us gather also from the words of the Apostle by what Mystery Christ rose from the dead: Remember that Christ has risen from the dead, of the seed of David. Here he teaches that the death and resurrection are due only to the Dispensation by which Christ was flesh.
Source: On the Trinity (New Advent)