Jesus dealt with that in
John 3 and Paul dealt with that in the book of Romans.
And this is a very, very important point that way too many Christians miss.
In Romans Paul is writing to both believers and unbelievers in Rome. Many of the Jews who fled Jerusalem and the Holy Land during the persecution ended up in Rome.
Paul addresses those Jews in Chapter 10 when he says to them in verse 5 that Moses writes about the Law, which the Jews were still trying to keep.
Then in the following verses he teaches them about Jesus.
Then in verse 9 he says, "that if you will confess with your mouth that Jesus is Lord . . . "
To the Jews, the world "Lord" brought to their minds Yahwah. The Jews believed in God the Father. They even believed in God the Holy Spirit in a limited way. But they did NOT believe in Jesus, God the Son!
Paul is saying that all they lack is to believe that Jesus is Yahwah, the Lord of the Old Testament, and they will be saved.
Paul goes on to say, "and believe in your heart that God raised him from the dead, you will be saved."
Believing in Jesus's triumph over death is the proof that He is Messiah, the Lord.
Paul recognized, in 60 AD, that the Jews worshiped another God and not the true Triune God.