From YOUCAT, fact 75:
75: Why do Christians address Jesus as “Lord”?
“You call me Teacher and Lord; and you are right, for so I am” (Jn 13:13). [446-451, 455]
The early Christians spoke as a matter of course about Jesus as “Lord,” knowing that in the Old Testament this title was reserved as a form of addressing God. Through many signs Jesus had shown them that he had divine power over nature, demons, sin, and death. The divine origin of Jesus’ mission was revealed in his Resurrection from the dead. Thomas confessed, “My Lord and my God!” (Jn 20:28). For us this means that since Jesus is “the Lord,” a Christian may not bend his knee to any other power.