In the final episode of Good Omens, Aziraphale and Crowley are captured by the forces of Heaven and Hell and put on trial for treason.
Both are sentenced to death, Aziraphale by hellfire and Crowley by holy water, but to the surprise of their masters, they are not hurt by these.
Belzeebub even says,
He became native
He is not one of us anymore.
We later see that they had actually switched places, not become native.
What does 'became native' actually mean?
We learned earlier in the season that Crowley was an angel that fell. Can a demon rise up and become an angel? Is that what becoming native means, or that spending too much time on earth means you become native?