I think that the crucial issue with the diffent denominations are not the CORE of their beliefs as much as it is that they focus on interpretations over scripture.
While the Trininy is NOT scripture, for no where is it plainly stated that, neither is Jesus being Michael, neither is not celebrating holidays or Bdays, neither is 1914 or any date for that matter.
Undertsanding the difference between what is scripture and what is interpretation would help many people go a long way to getting along better and being more open to dialoge.