13 So the Dispensation of the great and godly mystery makes Him, Who was already Father of the divine Son, also His Lord in the created form which He assumed, for He, Who was in the form of God, was found also in the form of a servant. Yet He was not a servant, for according to the Spirit He was God the Son of God. Every one will agree also that there is no servant where there is no lord. God is indeed Father in the Generation of the Only-begotten God, but only in the case that the Other is a servant can we call Him Lord as well as Father. The Son was not at the first a servant by nature, but afterwards began to be by nature something which He was not before. Thus the Father is Lord on the same grounds as the Son is servant. By the Dispensation of His nature the Son had a Lord, when He made Himself a servant by the assumption of manhood.
Source: On the Trinity (New Advent)