There is a Baha’i metaphor of ‘Progressive Revelation’ as a sort of relay race with fire.
The lamp of revelation is passed from one runner to another, as one revealed religion succeeds another in its proper time. So Isa was a true prophet for his time, but passed the lamp on to Muhammad, and likewise Muhammad was a true prophet for his time, but it came to an end when he in turn passed the lamp of revelation on to Baha’ullah, bearer of the proper revelation for his time (which is ours).
Needless to say, when I first encountered this metaphor I recoiled from it strongly.
It is somewhat disingenuous to claim to cleave root and branch to the essential unity of all religions, but have this picture of one ineluctably and properly giving way to the next. Any Jew-slaughtering Crusader in the Rhineland, up to his knees in gore back in 1096, would surely agree. ‘Aye, the revelation of Moses was good enough then, but times have moved on, eh?’ (Hack, slash, pitiful cry for mercy, stomp, splat)
I think most any observer of history would agree that the vast majority of the achievements of Christianity, for good and ill, and the most complete and self-consistent expositions of Christian thought., were after the life of Muhammad. Most observers of history would probably agree with me that the greatest achievements of Judaism, and the most complete and self-consistent expositions of Jewish thought, were after the life of Christ. It did not seem credible that these religions would persist and develop and achieve things, if the light of revelation had ‘moved on’. There’s that bit of Acts I quote all the time: ‘If their purpose or activity is of human origin, it will fail. But if it is from God, you will not be able to stop these men, you will only find yourselves fighting against God.’
My counter-metaphor was of plants in a garden. Actually, I think my first metaphor was of a glove, but that did not turn out so well. No religion owns the light. It is not passed from one to another. They are all down here, striving toward the light, which is up there. Each revealed religion is not meant to stay the same. Each revealed religion is planted like a seed, and is meant to grow and develop. Each religion is nourished by and grows towards the same sun; each religion is composed of many parts, most of which are useful and necessary, some of which may be diseased or superfluous. They are, all of them, clearly still a long way from the sun. It will be valid to say that some of them are, on average, closer to the source of light; it will be valid to say that parts of each one are closer or further away from the source of light. It will be probable that bits of different plants will actually be closer to one another than they are to different parts of the same plant. These are all bits of the metaphor that are meant to be there.
Does that sound dreadfully relativist and wishy-washy? Maybe it is, a bit. But implicit in it is the idea that there is an objective scale by which we can say one religion is on average better than another, even if we don't have access to that objective scale. And that some bits of each religion are objectively functional and good, and others are dysfunctional and bad.