The life of a Christian is fundamentally interior. During our pilgrimage on earth, we do not see the Lord, except behind veils. (The veils are the sacraments, the Scriptures, other people.)
So we obey an invisible Master. Our exterior actions flow from our interior communion with Jesus Christ.
To the outside world, we appear to have a secret source of guidance and strength.
Many people admire this about Christians–that we have this mysterious depth.
Others fear that “invisible Masters” produce fanatics.
So: What is the difference between a fanatic and a saint?
A fanatic serves a misconception of God. The saint serves an invisible Master who actually is God.
Just because something is interior doesn’t mean it is purely subjective.
It is not fanatical to believe the Creed of the Church. There is a great deal of evidence in favor of believing. It is perfectly reasonable to believe that Jesus Christ is God.
That said, we must always strive to purify our interior lives. I could do wrong because of my own false ideas about the Lord–and all the while I tell myself “it is God’s will.” Our interior vision is never as clear as it should be. We clarify it by prayer and penance.
But we pray and do penance precisely in order to align our interior lives with the truth. There IS truth and falsity when it comes to religion, devotion, prayer, etc.
The fanatic serves a false god. A Christian serves the Truth. A reasonable person can tell the difference.
Father Mark White Blog 
Those are the key words-A REASONABLE PERSON.
Our culture decries Christianity then simplifies it as fanaticism.
I am thinking about how a person, who doesn’t believe in God, can call someone that does believe a fanatic? 😕
As you stated, “A fanatic serves a misconception of God.” Wouldn’t that imply that the nonbeliever does, in fact, believe in God and that they have the real conception of God?
I have heard many atheist use the word ‘fanatic’ when describing Christianity.
p.s. This is my third attemp at writting this comment. I lost the other two when I hit the wrong key and now I am confused as that what I wanted to say. 🙄
Anyhow, I enjoy reading your post.