The earliest creeds and confessions of the Church are those made by the congregations filled with the people that saw the Lord. When Paul was writing, a great part of the five hundred that has seen Christ post-resurrection was still alive, giving us the best accounts possible of what the Crucified left His Church.
I believe that the phrase ‘Jesus is Lord’ and the like, is derived from one of the most heavily quoted passages in the Old Testament, Joel 2.32:
And it shall come to pass that everyone who calls on the name of the LORD shall be saved. For in Mount Zion and in Jerusalem there shall be those who escape, as the LORD has said, and among the survivors shall be those whom the LORD calls. (Joel 2:32 ESV)
This was the very first passage quoted by Peter during the very first sermon given after the Ascension of Christ. (Acts 2.16). Joel 2.32 became heavily quoted by the Apostles, with the Apostles connecting the name of Jesus Christ to the name of YWHW in the Hebrew:
Let it be known to you all, and to all the people of Israel, that by the name of Jesus Christ of Nazareth, whom you crucified, whom God raised from the dead, by Him this man stands here before you whole. This is the “stone which was rejected by you builders, which has become the chief cornerstone.’ Nor is there salvation in any other, for there is no other name under heaven given among men by which we must be saved.” (Acts 4:10-12 NKJV)
The earliest creeds and confessions was the simplest and the highest Christologically speaking.
Therefore I make known to you that no one speaking by the Spirit of God calls Jesus accursed, and no one can say that Jesus is Lord except by the Holy Spirit. (1 Corinthians 12:3 NKJV)
that if you confess with your mouth that Jesus is Lord and believe in your heart that God has raised Him from the dead, you will be saved. (Romans 10:9)
Your attitude should be the same that Christ Jesus had. Though he was God, he did not demand and cling to his rights as God. He made himself nothing; he took the humble position of a slave and appeared in human form. And in human form he obediently humbled himself even further by dying a criminal’s death on a cross. Because of this, God raised him up to the heights of heaven and gave him a name that is above every other name, so that at the name of Jesus every knee will bow, in heaven and on earth and under the earth, and every tongue will confess that Jesus Christ is Lord, to the glory of God the Father. (Philippians 2:5-11 NLT)
And this gospel he had announced beforehand through his prophets in holy writings — It is concerning his Son, Jesus Christ our Lord, who became flesh through the seed of David, And who, by his divine spirit, has been declared with power to be the son of God, by the resurrection from the dead: Through whom we receive grace and commissioned as an apostle in order to bring about the obedience caused by faith among all nations for his name, (Romans 1:2-5 CTV-NT)
And without controversy great is the mystery of godliness:
God was manifested in the flesh,
Justified in the Spirit,
Seen by angels,
Preached among the Gentiles,
Believed on in the world,
Received up in glory.
(1 Timothy 3:16 NKJV)