Religions and Corporations
There are four major religions in the mod: Catholicism, Orthodoxy, Islam, and Protestantism. Catholicism and Orthodoxy are relatively similar. The Islamic nations start out which some significant advantages in research, but these wane as the mod progresses. The Protestant reformation is triggered as soon as one civ researches Printing Press. At that point, all Catholic Civs must choose which side of the reformation they will join. The AI choses somewhat randomly (based on history), while the human is free to chose. Judaism is a minor religion. It spreads much more slowly, and has no missionaries to spread it manually. It is useful for the gold it can bring in, but non-state religions in your cities decreases your stability. You may want to use the new religious prosecutor (aka the Inquisition) to remove heretics from your cities. See the Religion Overhaul for more information.
There are new corporations. The first three available are military orders and allow special units. The later four are merchant/bank organizations and act like regular corporations. Eventually, the military orders should obsolete. The different military orders compete for resources, and thus with each other. You will only be allowed one in each city. The three banking families also compete pairwise, so you cannot have more than one of these per city. Hanseatic competes with Teutonic, Hospitaller, and Medici. Bank of St. George competes with Knight Templar.