The NT writers do not argue the rightness of making this daring declaration, identifying Jesus as God, for there is no need for them to do so. Paul defended gospel of justification by grace through faith because it was challenged. Jesus is Lord, as this would allow a conversion that involves trusting him as Savior without surrendering to him as Lord. Jesus is always Lord before he is Savior. They are one, indivisible.
Lordship implies salvation, announces salvation.
Here is the symbol of victory over all forces of evil. Salvation is possible because of this victory, because of his Lordship. There is no salvation without Lordship. The idea that Jesus is Lord and the idea that Jesus saves are virtually synonymous. In the first century, kurios could be a merely respectful designation attributed to landowners.