WARNING: The following contains spoilers for the Fire Force manga up to and for chapter 290, "Piercing Hope", by Atsushi Ōkubo. Available for streaming on Comixology.
The imposing figure of the white-clad Evangelist has inspired many fans to theorize about the true nature of the series' main antagonist and whilst a lot of knowledge has been gained regarding the being's religious doctrines, barely any concrete information has been revealed about the character's identity so far. However, as the narrative reaches its ultimate climax, recent chapters have included revelations that are finally starting to unravel the mystery that is the Evangelist.
With the world of Adolla being exposed as a culmination of humanity's collective thoughts and perceptions, alongside some lengthy exposition from Haumea, arguments can easily be made for the Evangelist to be a representation of humanity's most primal evil, the Devil.
The Last Judgement of Shinra Kusakabe
Furthermore, chapter 233 ("Guardian angel",) revealed that Shinra Kusakabe was born from an immaculate conception and is actually a projection of humanity's concept of a Jesus-like savior. His little brother Sho was also born in a similar fashion, but as a "guardian angel" meant to ensure Shinra stayed on the right path. Therefore, as the two gear up to face Haumea and the remaining white-clad members, it is far more logical for the Evangelist to represent the concept of the Devil as a clear opposition to Shinra's "savior" role.
In chapter 287 "The Holy Woman of Despair", Sho describes the Evangelist as a materialization of all human's collective desire and Haumea elaborates by saying that humans evolved to ultimately desire their own extinction. Although on the surface this seems to dispute all theories surrounding the Evangelist having a specific identity, in actuality the method with which humanity expects its extinction to occur is not specified and it would most likely be either at the hands of the Devil or through God's judgement. The Evangelist is most probably the prior masquerading as the latter, evidenced in the story by Adolla repeatedly being compared to Hell, even by those on the Evangelist's side. The similarities between Adolla and Hell can't have been driven by mankind's perception, as the existence of the realm of Adolla is not known by the general public and is kept a guarded secret.
Whether intentionally on the part of Ōkubo, or just as a consequence of the sheer amount of religious iconography within Fire Force as a whole, the Evangelist has become the perfect representation of the Devil manipulating mankind from behind the scenes.