:::There is no way to know what a Protestant believes based on his or her denomination. Protestantism is anything goes. You never know what you are dealing with.:::
There are some that I have met, including a number here that have reasonably stable belief systems. But the majority of those that I have personally interacted with, and those denominations that I have gone through their online doctrines, tend to variability, often extreme.
Remember, there are Liberal Protestant churches out there just as there are Liberal Catholic churches out there.
But these variations cross denominational lines. Protestant sects are rleigious parties akin to the Pharisees and Sadducees, and not to "churches." Thus you have about 10% Southern Baptists who are "Reformed" (Calvinist) just as as Presbyterians who can be Calvinist or closer to Episcopelians in their views. Yet Calvinist Baptists who deny the free will have no issues with non-Calvinist Baptists who believe in free will and worship together.
Protestant "theology" is personal opinion based on reason. But faith based on reason is man-made naked rationalism. Which is why logic plays such a prominent role in many of these "theologies," especially in legalistic Calvinism.
Protestants belong to denominations based on personal and politcial factors as well. I know a couple in Virginia who changed from a Methodist church to a Presbyterian (USA) church because the spouse's ex-husband still goes to the Methodist church! And the current husband used to be Pentacostal and has no problems now sharing his divergent theology with Prebyterians, nor does his Methodist wife ("raised Methodist, bene Methgodist all my life" to quote her) have probles being with Presbyterians either.
One of the Freepers on this forum stated that he would switch to a Calvinist church in a heartbeat if one opened next to his (even though he doesn't believe in 'baby splashing'—a Baptist euphamism for infant baptism)
This shows that there is no "church" per se, but different assemblies to which people flock for many reasons, mainly personal. They also go "church-shopping" a lot. It's always about "me" and what fits "my notion of God" as "I understand it." So, people look for like-minded communtiies and call them "churches." Which is what this whole thread is really about. :)
Some Protestants have stable views just as some people have stable views on different subjects, say a cosmologist on the necessity of "dark energy" to exist in order for the equation to work. But in either case it's a man-made system, stable as it may be.
However, If you compare the views of two or more such individuals you will find divergence on key issues (Christology, Trinity, free will, etc.) even though thye may worship in the same denominational community thye call a "church."