Similar to Boris, I also saw this section of What the Thunder Said as a commentary on religion, specifically that of Christianity. The transcendence between the living and the dead here immediately made me think of Jesus Christ as a resurrected and reborn body. Boris quotes Jesus from the Book of Revelations stating:
'I am the first and the last. I am he that liveth, and was dead; and behold, I am alive for evermore, Amen; and have the keys of hell and of death
While Jesus "[lives] for evermore," those who "were living are now dying." I took particular notice of Eliot's use of the state of living in the past tense of "were." The "we" he references here do not appear to yet be fully dead and yet, Eliot references their existence in the past tense, as if they're caught between two worlds or states of being. Boris saw this first line of "He who was living is now dead" as a reference to Jesus's reincarnation, but in the reverse. A Jesus who never transcended that barrier between that stage between life to death and then to rebirth back to the living. Following this interpretation, the following two lines "We who were living are now dying/With a little patience" can be seen as a drawing out of the mortal life or even the more uncertain stage that exists in the interim of life and death (some form of purgatory?). The "little patience" that plagues those who lived but are still actively "dying" could be a result of the lack of Jesus ever being reborn. In the absence of Jesus returning to life, the dead must remain and the living must continue to live, terminating that cycle we've Eliot emphasize throughout TWL. While I, personally, am not fully committed to this interpretation, I do find it to be an interesting take on Eliot challenging the conventions of religion and how its viewed in the modern world.