Mar 5:36 Be not afraid, only believe.
Alternative: Fear not, only believe.

Christ sees fear as the opposite of belief. Fear is usually seen as the emotion reaction to the unknown. In all of Christ's words, fear is always the failure of believing in God, his rules, and plan. Christ understood human fear, but consistently says that it is not necessary. There is nothing to fear in life, not even death. Fear comes from belief in what we see rather than the word of God.

The hard thing is to "only" believe, but the Greek word for "only" has an additional meaning that we don't have in English. It means "one above all others," so belief only has to be the foremost of our feelings. So, while we can have many conflicting feelings, the goal is to put our feeling of faith in God above all the others.

This isn't the only time that Christ uses the term for "belief" with the term for "only." He did it in Mat 21:21 describing his power to wither a fig tree. When believe can become the strongest of our feelings, miracles become possible. We also see the word for "only" in Luk 8:50, which is another version of this same story. This is therefore another example of how two Gospel writers pick the same rarely used word to describe the same event. From this, I deduce that this exact word is important.

UPDATE: Fear not, only believe. Just read this post while I was thinking about how the sun has been cooling during a period when people fear global warming. What if God set is up the universe so as we create more greenhouse gas while transitioning to future technologies, the sun grows cooler to protect us.

"Afraid" is from phobeô (phobeo), which means "to be put to flight," "to fear," and "to be afraid."

"Only" is from monôs (monon), which means "alone," "solitary," "only," "one above all others," "made in one piece," "single," and "unique."

"Believe" is from pisteuô (pisteuo), which means "to trust, put faith in, or rely on a person," "to believe in someone's words," "to comply," "to feel confident in a thing," and "to entrust in a thing."