The Apostle Paul in Romans argues that a Christian cannot rightly call themselves a disciple unless they both believe and confess that ‘Jesus is Lord’.
9 If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. 10 For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved. 11 As Scripture says, “Anyone who believes in him will never be put to shame.” 12 For there is no difference between Jew and Gentile—the same Lord is Lord of all and richly blesses all who call on him, 13 for, “Everyone who calls on the name of the Lord will be saved.”
14 How, then, can they call on the one they have not believed in? And how can they believe in the one of whom they have not heard? And how can they hear without someone preaching to them? 15 And how can anyone preach unless they are sent? As it is written: “How beautiful are the feet of those who bring good news!” (Romans 10:9-13)