Any religion such as Christianity has certain core beliefs that define that religion. How do we Christians determine the validity of the doctrines we hold?
We have no evidence for most of our doctrines. While we have very good evidence that Jesus was a historical person and that he was killed for his teachings, we cannot prove Jesus was God and that he died for our sins. We must take most of our doctrines on faith.
We base our doctrines upon what the Bible says. How do we know our doctrines are correct? How do we know we have correctly interpreted the Bible? One way is to learn from others. Roman Catholics depend upon the pronouncements of its Popes and councils. Protestants depend upon the beliefs of the denomination or church to which they belong.
How do we know our church’s doctrines are correct? Do we take that on faith? Is it valid to have faith in fallible humans? On some doctrines, there are disagreements within the Christian faith—that is why we have so many different Christian religions and denominations. For example, some Christians debate the validity of the doctrine of the trinity.  This debate is based upon different interpretations of what is said in the Bible. So who decides which interpretation of the Bible is correct? As we have stated many times in this blog, we are finite. Being finite means while the Bible is infallible, our interpretation is not.
We Christians agree we are finite beings yet when it comes to our doctrines, we act as if we were infallible. Why?
 Anthony F. Buzzard, and Charles F. Hunting. The Doctrine of the Trinity. New York: International Scholars Publications, 1998.