We cherish freedom and choices, especially here in the US. But Paul presents a no-choice ("one" only) lifestyle: live in the manner worthy of your calling. No other way than Jesus? How offensive! Boring! Intolerant! Those who slander Christianity like this don't understand true freedom: not individualism, but rather a conscience owned by Christ. Ironic? Perhaps. But life in any other manner is, by comparison to Christianity, so poor as to be a non-choice.
Chapters 1-3 of Ephesians lay the basis for the "therefore" in 4:1, preceding these instructions. Briefly: you were dead in sin; God knew you before the womb; He has resuscitated you. Now ask: why does mankind exist? Answer: Christ has ransomed all mankind; they are now to walk in way of Christ Jesus. One Lord (Jesus Christ), one faith (the same), one Baptism (into Christ), one Father of all (the Father).
So your sinful self finds it hard to walk in Christ? The Old Adam tells you that there are other options? To refuse the means of grace? Answer him with the truth: he is death, dead. Not an option. Turn now to the work of God: "to believe in Him whom He has sent." Walk in His way thus: live your life to God and others; take a nap when your body tires; pray for your brothers and sisters.
Christianity isn't a bad option. It's the best and only real option!
Soli Deo Gloria.