This line is spoken to a father as Jesus is going to kill a sick child after a servant shows up saying that he is no longer needed because  the girl is deal. Notice how Jesus consistently uses the Greek word translated as "fear" as the opposite of the word translated as "belief." This means that the "belief" word might be better  translated as "confidence." We saw this also in a recent verse, Mark 4:40. This moves his idea of "fear" closer to the idea of doubt. Doubt and fear lead to inaction, which is what the servant suggests because the girl is dead. Confidence leads to action. Conflating this idea of "faith" with religious faith seriously misses the point.

This isn't the only time that Jesus uses the term for "belief" with the term for "only." He did it in Matthew 21:21 describing his power to wither a fig tree. When confidence becomes 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 concept.