In Jesus’ day, the word lord was often used as a title of respect toward earthly authorities; when the leper called Jesus “Lord” in Matthew 8:2, he showed Jesus respect as a healer and teacher (see also Matthew 8:25 and 15:25).
However, after the resurrection, the title “Lord,” as applied to Jesus, became much more than a title of honour or respect. Saying, “Jesus is Lord,” became a way of declaring Jesus’ deity. It began with Thomas’ exclamation when Jesus appeared to the disciples after His resurrection: “Thomas said to Him, ‘My Lord and my God!’” (John 20:28). From then on, the apostles’ message was that Jesus is Lord, meaning “Jesus is God.” Peter’s sermon on the Day of Pentecost contained that theme: “Let all Israel be assured of this: God has made this Jesus, whom you crucified, both Lord and Messiah” (Acts 2:36). Later, in Cornelius’s house, Peter declared that Jesus is “Lord of all” (Acts 10:36).
To say that “Jesus is Lord” means that Jesus is God. Jesus has “all authority in heaven and on earth” (Matthew 28:18). He is “our only Sovereign and Lord” (Jude 1:4). He is, in fact, the Lord of lords (Revelation 17:14).