To be honest, Bonhoeffer isn't even all that good (says a kid who has no authority for making such claims haha). But he's not beyond making some excellent points! I'll spare you him and give you my clarification. There is quite a lot at play in your question. If I parse it rightly, there are themes of: new obedience (part of sanctification in the narrow sense), eschatology (end things; now and not yet), comparative religions (you make it sound like a gradient?), the role of the human will, etc. I'll focus on two: (1) eschatology in the now and (2) new obedience.
(1) Eschatology in the now. There's a hymn called "Heaven is my home". This is wrong. Heaven is not our home. Our home is the "new heavens and the new earth" (Rev. 21). “I believe in the Holy Christian Church, the communio sanctorum, the forgiveness of sins, the resurrection of the body, and the life everlasting.” I confess that obedience and prosperity are undoubtedly limbs of the Christian corpus (to repeat myself from earlier). But again, I assert, obedience and prosperity are not the heart. When I deny that the most general (which I interpret to be synonymous with fundamental, if that’s not too far afield?) definition of Christianity is not human prospering, I am not aligning myself with an escapist theology solely focused on getting justified and then - forgive me - getting the hell out of here, everything else be damned. Rather! I’m saying justification (which has both eschatologically present and future implications) is the central datum and must therefore inform our understanding of human prosperity and new obedience. Speaking eschatologicaly, justification for Christ’s sake will invite us to try and live in the here and the now as if we were in the new heavens and the new earth. That is: to live with (very imperfect) eyes that see everything as grace; given by God (through many, various, and mysterious means and without our meriting it in the eyes of the Almighty). Which leads into...
(2) New obedience. The free justification (that is, the promise of an eternally right and just relationship with God in the new heavens and the new earth) won by Christ, that foundation upon which the Church stands or falls, enlivens and enlightens us in both the here-and-now and into eternity. The overwhelming majority of what enlivening and enlightening into eternity, the new heavens, etc. will look like and mean are not given and so I must resist speculation. However, in this life, being certain of one’s stance in the eyes of God (just/right/adopted/befriended by his kindness in Christ) enlivens a Christian to love her neighbor as Christ has loved her. Thus, a Christian seeks the prosperity of his neighbor (and himself, with neighbor in mind), understanding that to do so is to imitate Christ (Gal. 2:20). Not because she or he is convinced Christianity leads to the most prospering and best obedience, but because they are free to seek obedient lives and the prosperity of others (using their knowledge and talents, attempting to make the best choices), knowing their eternity is secure in God and no longer resides in their decisions (Col.3:3).
Maybe the Christian rendition of morality more than any other moral system does lead to a more prosperous and just society. But I wonder what metric you would use to measure it, what evidences you would have to take into account, and why it would be important to you. To that last one, Christians are just as sinful as everyone else and are thus just as in need of forgiveness. The only difference is that God has convinced them of their need and provided for them. Thanks be to God he came to save sinners and not the righteous. (Lk. 18:9-14)
I do wonder if the argument of these assertions was perspicuous, internally logical, and understandable? Let me know if you’d like clarification! Your question was excellently put and it got me to thinking. And of course, I’m open to correction (insofar as anyone is and isn’t, I suppose) and am happy to reflect on whatever you’d like to share or ask. My above comment by no means exhausts the topics. But I think they accurately and generally reflect the assertion that justification in Christ is the sole foundation which makes Christianity important (and distinct from all religion/s); everything else is peripheral (not to minimize it unduly).