What are the most controversial and heatedly debated doctrines in Christianity? To be more precise, I'm looking for examples of doctrines which are:
- objectively popular, i.e., there is a significant amount of Christians who believe in this doctrine.
- objectively controversial, i.e., there is a significant amount of other Christians who reject this doctrine for different reasons, and it is not uncommon for debates to ensue when the doctrine or its rejection are brought up in the presence of an opposing party.
Answers that substantiate claims of controversy with objective statistics would be preferred.
Note: for the purposes of this question, to disambiguate the meaning of significant amount of Christians let's assume that if a doctrine is defended by at least 1 million Christians and rejected by at least 1 million other Christians, then it is objectively popular and controversial.
Note 2: given the potential combinatorial explosion of comparisons between different denominations, I would suggest that answerers define their own 'controversy scores' to help them rank controversies. One option could be to sum the numbers of Christians for and against the doctrine, or use the minimum between the two, etc. Another criterion to rank controversies could be the degree of importance attributed to the doctrine by the defending party (or to its rejection by the antagonist side). For example, if the doctrine or its rejection is considered essential for salvation by either party, that should definitely add bonus points to the controversy score. An answer listing the "top 5" or "top 10" doctrinal controversies according to a criterion thus defined would be perfectly fine in my opinion.
Note 3: another way to identify (and justify) the existence of doctrinal controversies is by finding examples of public organized debates between notable apologists representing the different positions in dispute. For example, see Debating the Eucharist // Cameron Bertuzzi vs. Matt Fradd.