Faith is one of the most misunderstood words in the Christian vocabulary. Many people hear it as "believing things that are not true" or "being very, very sure."
Neither is what the Bible means.
Faith is trust, placed in someone
Biblical faith is not a feeling of certainty you manufacture. It is trust — directed at a person who has proved reliable.
You use this kind of faith constantly. You sit on chairs without testing them. You take medicine you cannot chemically verify. That is not irrationality; it is trust based on good grounds.
The object matters more than the amount
Jesus said faith the size of a mustard seed is enough. That is a strange thing to say if faith works by volume.
It makes perfect sense if faith works by object. A little trust in something completely reliable will hold. Enormous confidence in thin ice will not.
So the question is never "do I have enough faith?" It is "who am I trusting?"
Faith and doubt can share a room
The father in Mark 9 says, "I believe; help my unbelief." Jesus does not rebuke him — He answers him.
You do not need to eliminate doubt before you act. You need to act in the direction of what you trust, doubts and all. That is what faith has always looked like.