Fludd was a licensed London physician, a Fellow of the College of Physicians in good standing, who used his considerable private means to publish enormous, lavishly engraved folios setting out a complete cosmology of macrocosm and microcosm, the human being as a scale model of the universe, ordered by the interplay of light and darkness, with the same proportions governing both.
He defended the Rosicrucian manifestos in print in the 1610s while denying membership of any brotherhood, and there is no evidence he was in one, because there is no good evidence any existed to be in.
His lasting interest is the public quarrel with Johannes Kepler, conducted in print from 1619 to the early 1620s, because it is the clearest surviving statement of a parting of the ways. Both men held that the cosmos was ordered by harmonic proportion. Kepler's objection was about method: Fludd's harmonies were symbolic, derived from correspondence and analogy, and did not have to answer to measurement, whereas his own had to match observed planetary data or be abandoned. Fludd's counter was that Kepler had confined himself to the outer husk of quantity and missed the formal reality that the numbers signify.
Kepler's side of that argument became science and Fludd's became esotericism, and the split is nearly this clean. It is worth noting that Kepler himself is no simple champion of modernity here: he retained the harmonic and Platonic programme throughout, and republished his nested-solids model of the planetary distances in a second edition in 1621, more than twenty years after the Mysterium Cosmographicum first appeared and long after his own laws were in hand. He did not abandon it.