An actual devil materialized in this world and his power was passed through the ages in the same lineage ---> early Marshalls ---> Xebec ---> BB
A spirit of liberation emerged from the prayers of enslaved moon people, and his power was passed through the ages ---> Nika ---> Joyboy ---> Luffy
They are one the antithesis of the other, and are bound to clash again and again, until finally one of them prevails and reshapes the world in his image.
Until then, on either side of the spectrum, factions will employ powers that mirror both the devil's and the liberator's power for their own agendas, from enslaving the world (IMU) to trying to liberate it (Dragon), but none of their struggles are definitive, merely small episodes in the grander, cosmic war at the core of the series.