Jesus is Lord” is the original confession of faith, test of orthodoxy, bond of fellowship, and death gasp. This confession is the test of the New Covenant in the same way that “Hear, O Israel, the Lord our God is one…” was the confessional test of the Old. It is a confession, not of belief, but of allegiance.
This only sounds alarming to you if you persist on that most modern of mistakes, which is defining a relationship in terms of belief content, instead of the way Jesus consistently defined it, which was by allegiance. This was so, so Hebrew of Him. Belief-content is all well and good, and the NT does instruct us on what to believe, but belief-content, as it is propagated even among the protestant cognoscenti, is a subjective checkbox, whose purpose is subjective assurance, to silence a subjective and nattering conscience.
Jesus asked for objective allegiance, which is lived out — I hesitate to use the maligned word — existentially. In external situations. Like when the emperor is going to burn you alive depending on who is your lord.
Our modern sensibility immediately wants to seize on that one simple sentence and break it down. We want to argue, analyze, and exegete it out. This impulse, which would be foreign to the NT believers, is a feature of leisure more than anything else. We have so few existential tests of that Jesus-allegiance that our psyches feel the need to do something and what they do, naturally, as they drift along, is natter and niggle.
Hush. Jesus is Lord. Live it.