69 The Apostle then looking in us for the righteousness which is of Faith, cuts at the root of incredulous doubt and godless unbelief. He forbids us to admit into our hearts the cares of anxious thought, and points to the authority of the Prophet's words, Say not in your heart, Who has ascended into heaven ? Then He completes the thought of the Prophet's words with the addition, That is to bring Christ down. The perception of the human mind cannot attain to the knowledge of the divine: but neither can a reverent faith doubt the works of God. Christ needed no human help, that any one should ascend into heaven to bring Him down from His blessed Home to His earthly body. It was no external force which drove Him down to the earth. We must believe that He came, even as He did come: it is true religion to confess Jesus Christ not brought down, but descending. The mystery both of the time and the method of His coming, belongs to Him alone. We may not think because He came but recently, that therefore He must have been brought down, nor that His coming in time depended upon another, who brought Him down.
Nor does the Apostle give room for unbelief in the other direction. He quotes at once the words of the Prophet, Or Who has descended into the abyss, and adds immediately the explanation, That is to bring Christ back from the dead. He is free to return into heaven, Who was free to descend to the earth. All hesitation and doubt is then removed. Faith reveals what omnipotence plans: history relates the effect, God Almighty was the cause.
Source: On the Trinity (New Advent)