There are people who just believe or have faith in the material world. You have to believe in the world. You do not know everything about this world and you believe in what you do not know. There is no need to believe what you know. You have to believe what you do not know. You do not know about anybody. You do not know about yourself, your wife, children, husband, your parents, or anybody. Do you think you know anything about them? If you think you do, you are mistaken. You know something about them, partially. 

Faith in your Self, faith in the world and faith in something we do not know, the Divine. You can start from anywhere. Any one point will lead to all three; all three come together. When doubts arise in you, keep a fast, meditate, do pranayama and prayers, and see the shift that happens and the doubts vanish. There’s a shift in your consciousness.