1 The Apostle in his letter to the Ephesians, reviewing in its manifold aspects the full and perfect mystery of the Gospel, mingles with other instructions in the knowledge of God the following: As ye also were called in one hope of your calling; One Lord, one faith, one baptism, one God and Father of all, and through all, and in us all. He does not leave us in the vague and misleading paths of an indefinite teaching, or abandon us to the shifting fancies of imagination, but limits the unimpeded license of intellect and desire by the appointment of restraining barriers.
He gives us no opportunity to be wise beyond what he preached, but defines in exact and precise language the faith fixed for all time, that there may be no excuse for instability of belief. He declares one faith, as he preaches one Lord, and pronounces one baptism, as he declares one faith of one Lord, that as there is one faith of one Lord, so there may be one baptism of one faith in one Lord. And since the whole mystery of the baptism and the faith is not only in one Lord, but also in one God, he completes the consummation of our hope by the confession of one God.
The one baptism and the one faith are of one God, as they are of one Lord. Lord and God are each one, not by union of person but by distinction of properties: for, on the one hand, it is the property of Each to be one, whether of the Father in His Fatherhood, or of the Son in His Sonship, and on the other hand, that property of individuality, which Each possesses, constitutes for Each the mystery of His union with the Other. Thus the one Lord Christ cannot take away from God the Father His Lordship, or the one God the Father deny to the one Lord Christ His Godhead.
If, because God is one, Christ is not also by nature divine, then we cannot allow that the one God is Lord, because there is one Lord Christ: that is, on the supposition that by their oneness' is signified not the mystery, but an exclusive unity. So there is one baptism and one faith of one Lord, as of one God.
Source: On the Trinity (New Advent)