“Lord” in English is used to translate the Hebrew word ‘adonai or of the Greek kurios. The Hebrew YHWH is usually translated “Lord”; See Yahweh (YHWH).
God's rule and authority as Lord is based on his creation and ownership of everything and everyone (Psalm 24:1–2). The Bible emphasizes God's complete power over nature by calling him the Lord over:
earthquakes, wind, fire (1 Kings 19:10–14);
stars (Isaiah 40:26);
beasts and sea monsters (Job 40–41); and
primeval chaos (Psalms 74:12–14; 89:8–10).
The later prophets taught that God is Lord or King of history because he guides the actions of people and nations (1 Kings 19:15–18; Isaiah 10:5–9; Amos 9:7). They also said he is the Lord of universal morality (Ez 25–32; Am 1:3–2:16). But he is especially the Lord of Israel. His expressed will is their civil and religious law and requires complete obedience (Exodus 20:2).
For Israel, God's power was a comfort when they were oppressed. It also gave them hope for the future. They believed in a triumphant Day of the Lord that would right its wrongs, punish those who oppressed them, and restore its glory (Isaiah 2:2–4, 11–12; 34:8; Ezekiel 30:1–5; Joel 2:31–3:1).
In the Septuagint (the ancient Greek translation of the Old Testament), the usual word for "Lord/lord/master" is kurios. In the Greek New Testament, kurios is also used for:
masters, husbands, and rulers (Matthew 25:11; Luke 14:21; Acts 25:26; 1 Peter 3:6)
God (Matthew 11:25; Hebrews 8:2); and
pagan gods (1 Corinthians 8:5).
When used for Jesus, kurios can mean:
a common title of respect (like “sir,” Matthew 8:2; 15:25);
a title expressing faith, reverence, and worship (Matthew 3:3; Luke 7:13; Acts 5:14; 9:10; 1 Corinthians 6:13–14; Hebrews 2:3; James 5:7)
It appears in phrases like:
“the Lord Jesus”
“the Lord’s Day”
“the Lord’s Table”
“the Spirit of the Lord” (who is also “Lord,” 2 Corinthians 3:17),
“in the Lord”
“from the Lord”
“light in the Lord”
“boast in the Lord”
Sometimes it is not clear whether God or Christ is intended (Acts 9:31; 2 Cor 8:21). The title is attributed to Jesus himself in John 13:13–14. In John 20:28, Jesus accepts the title “My Lord and my God!”
In the first Christian sermon, Peter makes Jesus’s lordship central to salvation (Acts 2:21). It seems that publicly saying "Jesus is Lord" was the main way to express Christian faith. It was also the basis for membership in the early church (Acts 16:31; Romans 10:9; 1 Corinthians 12:3; Philippians 2:11). However, this could become just a formal statement rather than sincere belief. That's why there are warnings in Matthew 7:21 and Luke 6:46.
From the beginning, calling Jesus "Lord" had deep meaning:
In common usage, “lord” reflected the slave system and implied the absolute power exercised by the master over the purchased slave. So Paul confidently explains the moral implications of Christian redemption (1 Corinthians 6:19–20; 7:22–23).
To Jewish minds, the title had royal and authoritative meanings related to the Messiah (Luke 20:41–44). This offended both Jews and Romans.
Politically, “Lord” was a title claimed by Caesar. So it is significant that Jesus is called "King of kings and Lord of lords" during the time of Domitian, when Caesar worship was required (Revelation 17:14; 19:16).
For Greek-speaking Jews living outside of Israel who were familiar with the Septuagint, and for gentiles who used the title “Lord” for their many gods, calling Jesus “Lord” was seen as blasphemy. This was especially offensive when it was linked to titles like “Son of God,” as well as acts of prayer, praise, complete devotion, and hope (1 Corinthians 8:5–6; Philippians 2:9–11; 1 Thessalonians 4:14–17). For all these reasons, showing deep respect to Jesus was not only spiritually important but also brought serious and immediate danger.