28 Now the Apostle asserts that those words in the Gospel, I and the Father are one, imply unity of nature and not a solitary single Being, as he writes to the Corinthians, Wherefore I give you to understand, that no man in the Spirit of God calls Jesus anathema. Perceivest thou now, O heretic, in what spirit you call Christ a creature? For since they are under a curse who have served the creature more than the Creator— in affirming Christ to be a creature, learn what you are, since you know full well that the worship of the creature is accursed.
And observe what follows, And no one can call Jesus Lord, but in the Holy Spirit. Do you perceive what is lacking to you, when you deny Christ what is His own? If you hold that Christ is Lord through His Divine nature, you have the Holy Spirit. But if He be Lord merely by a name of adoption you lack the Holy Spirit, and art animated by a spirit of error: because no one can call Jesus Lord, but in the Holy Spirit. But when you say that He is a creature rather than God, although you style Him Lord, still thou dost not say that He is the Lord. For to you He is Lord as one of a common class and by a familiar name, rather than by nature. Yet learn from Paul His nature.
Source: On the Trinity (New Advent)