Now, I find a smiliar division within Catholicism. There are three groups, basically. (1) Traditional Catholics who accept whatever is taught by the magisterium. (2) Free-thinking Catholics who remain in the church but disagree with the magisterium on some basic theological or perhaps policy issues. (3) It's estimated that at least 30 million Catholics have left the church in the US, some to become mainline liberal Protestants, others to become evangelical conservative Protestants, maybe the largest number to come inactive altogether. Very small groups have become Orthodox, Mormon, Jehovah Witnesses, Buddhist, etc.
Polling of Catholics, for example, has shown that roughly 57% believe that Christ is truly present in the Eucharist, while 43% do not believe that. That represents a very serious division, even if the 43% remain in the church. The majority seem to favor married priests, and they divide almost evenly re the ordination of women priests. Actually, Catholics as a whole seem more 'liberal' than evangelical Protestants. The majority voted for Obama, for example, while evangelical Protestants voted against Obama by a significant margin. Mainline Protestants were split, a slight preference for Obama. Remember that traditional political affiliation plays a role. The majority of Catholics have been traditionally Denocratic. Black Protestants voted about 95% for Obama, and Black Protestants are roughly 20% or a bit more of all American Protestants.
Back to the 'stickyest' issue. Protestants, almost by definition, emphasize individualism, freedom of conscience and belief. The evangelicals as a whole accept the Bible quite literally, including Adam and Eve forward. The mainstream Protestants have been heavily influenced by modern thinking that have difficulty with certain supernatural teachings. They agree on some things that set them off from Catholics. St. Padre Pio, canonized not that long ago, claimed to have levitated, bilocated and much more. All Protestants would likely regard that as untrue. And with Catholicism, if one thing is untrue the whole system can be undermined.
There are hugely controversial issues regarding the priesthood as well as social ethics. Mainline Protestants generally favor the ordination of women, less popular among evangelicals - although the highest percentage of women clergy has been within the Salvation Army. which is evangelical. Mainline Protestants are likely to allow abortions in certain cases - e. g., to save the life of the mother, certainly. Most mainline Protestants and evangelical Protestants don't have a problem with artificial birth control. They might argue that we have artificial hearts and limbs, etc. They also may say that the Catholic approved method and artificial birth control both aim at avoiding pregnancy - so? If we truly believe that God decides whether or not pregnancy occurs, it will happen regardless.
Back to the theme: Protestants, as stated in an earlier post, believe that 'their church' goes back to Jesus, but reform was needed when it went so far off track. Different denominations provide a wide selection. Millions of Protestants, of course, have no problem switching denominations if they move and find a church nearby that they like, or are inspired by a certain preacher, or one church offers a much superior Sunday School to another, etc. They don't really 'convert' to another denomination. Among mainline Protestants, especially, theological differences don't play a big role as most of them accept the same basic beliefs and don't worry about the details. Evangelicals tend to worry more about theology and its details, which is a major reason as to why they keep dividing.
More than enough sed. Hope it makes sense.