A leading expert in the New Testament, Professor James Dunn, comments on the use of ‘lord’ in Aramaic and Greek at the time of Jesus:
‘We need not doubt that the Aramaic mari underlies the Greek kyrie (vocative)…Mar was used of the first century BC holy man Abba Hilkiah, presumably in recognition of the charismatic powers attributed to him.
Moreover, ‘lord’ was largely synonymous with ‘teacher’ at the time of Jesus, and Jesus was certainly recognised to have the authority of a rabbi or teacher (Mark 9:5 etc). We can therefore say that the confession of Jesus as Lord was rooted within the ministry of Jesus to the extent that he was widely acknowledged to exercise the authority of a (charismatic) teacher and healer (cf. Mark 1:22,27).
Whether ‘Lord’ already had a higher significance for Jesus himself during his ministry depends on how we evaluate Mark 12:35-37:
‘While Jesus was teaching in the temple courts, he asked, ‘Why do the teachers of the law
say that the Messiah is the son of David? David himself, speaking by the Holy Spirit, declared:
‘ “The Lord said to my Lord:
‘Sit at my right hand
Until I put your enemies
Under your feet.”‘
David himself calls him “Lord”. How then can he be his son?’
Even if it contains an authentic word of the historical Jesus (as is quite possible) it need only mean that he understood Messiah to be a figure superior to David in significance and specially favoured by Yahweh. It does not necessarily imply that he thought the Messiah was a divine figure (Psalm 110 after all probably referred to the king).
From: Unity and Diversity in the New Testament: An Inquiry into the Character of Earliest Christianity pp. 53-54.
So Dunn recognises that the title ‘lord’ originally denoted a human being. As the term began to be used in pagan contexts as the Gentile mission spread, where it was well established as a title for the cult deity in the mystery religions (especially Isis and Serapis), and also in Emperor worship – “Caesar is Lord” – it becomes clear that a radical alteration of the meaning of the term occurred. Above all, St Paul advanced this change in meaning quite deliberately. He uses Old Testament texts that speak of Yahweh and applies them to Jesus (e.g. Paul’s Letter to the Romans 10:13). For Paul, it appears that ‘Lord Jesus’ had become a title of divinity.
In a profound sense, Paul founded the religion of Christianity we know today.