Robert Silverberg 
To Open the Sky [EPUB ebook] 

The Vorsters Were the blue-robed worshippers of the atom, symbolised by the Cobalt-60 reactors that glowed blindingly on every alter in their fast-growing churches.The Harmonists Were the green-robed heretics, a breakaway faction condemned as icon-adorers, who believed that their creed was truer to the code of Vorst, the Founder.At the beginning of the 22nd century, Earth colonies were established on Mars and Venus. But the ultimate dream — to travel to the stars — was still an impossibility. A few enlightened men believed that the Vorsters and the Harmonists could solve this seemingly insurmountable problem, if they could only forget their differences and work together. But the hatred between the two factions was too deep to be reconciled — until a conflict on Venus between a Vorster priest and his Harmonist opponent resulted in some totally unexpected developments . . .