Goirdano had two major faults, as far as the clerical authorities were concerned. Firstly he was a Copernican. Actually, that's not true: he was an uber-Copernican. While other astronomers were cautiously considering the possibility that the Earth might go round the sun, and were being frightened into recanting this heresy, Giordano Bruno had an SF vision.
In De l'Infinito, Universo e Mondi, he argued that the stars we see at night were just like our Sun, that the universe was infinite, with a "Plurality of Worlds", and that all were inhabited by intelligent beings... and that the same physical laws apply throughout.
His second fault was related to his first. Believing space to be infinite, and this world just one among many, he held that god was present in all things, and that all of creation was holy. He also - incidentally - held that women were of equal status to men. What a guy.