26 At the outset take note that this is not the order of the Apostle's teaching, for in that order the surrender of the Kingdom is first, then the subjection, and lastly the end. But every cause is itself the result of its particular cause, so that, in every chain of causation, each cause, itself producing a result, has inevitably its underlying antecedent. Thus the end will come, but when He has delivered the Kingdom to God. He will deliver the Kingdom, but when He has abolished all authority and power. He will abolish all authority and power, because He must reign. He will reign until He has put all enemies under His feet. He will put all enemies under His feet, because God has subjected everything under His feet. God has so subjected them as to make death the last enemy to be conquered by Him. Then, when all things are subjected unto God, except Him Who subjected all things unto Him, He too will be subjected unto Him, Who subjects all to Himself. But the cause of the subjection is none other than that God may be all in all; and therefore the end is that God is all in all.
Source: On the Trinity (New Advent)