Good. Bad. Both?

What makes up good theology? What makes up bad theology? How are we to know if theology is good or bad when one individual claims that God has led them through revelation to believing something and another person claims that God has led them through revelation to believing the exact opposite of the theology of the first person? In logic, it is impossible for both sides to be right and one of the two must be wrong. But what if by chance, both sides are wrong? It may be more profitable then to place higher emphasis on the reading of the holy scriptures above the theology of the next person to see what God has to reveal in your own understanding of theology.