First, we dont' have the "whole truth" just what God decided to reveal to us. Personally, I think the whole truth is beyond our comprehension so God gave it to us in a bite sized chunk to chew on until we get to heaven.
If you are a Protestant, most churches have some sort of written statement of beliefs that they hold to. Many use creeds such as the Apostles Creed. Some just have a statement of faith. If it isn't printed, you ask. Rarely are there surprises in a statement of faith.
And if one wishes to go deeper and fuller?
For example if I wished to find more about the divine nature that Sola Scriptura Protestants are in agreement on..
Same problem applies.
I'm curious if, as in your example, a Protestant Church holds to the Apostles' Creed, do they include the Communion of Saints in it?